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Financial Analysis puzzles, solved step by step

Puzzles
100
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All topicsAccounting flow riddles10Valuation and multiples riddles10Ratio and margin riddles8Cost of capital, leverage and rates8Compounding and time value8Mental maths8Probability and expected value9Working capital and cash riddles6Percentages and averages7Estimation and market sizing7Logic and counting brainteasers7Pricing, costing and unit economics6Data and statistics intuition6
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Showing 31–40 of 100
  1. 031A software company has EBITDA of Rs 800 crore and debt of 7x EBITDA. If its EV/EBITDA multiple falls from 10x to 8x, what happens to its equity value, and what does 7x leverage tell you about the sensitivity?Cost of capital, leverage and ratesCoreMoelis & CompanyNew York · 2026

    Try it first

    The multiple falls 20%, from 10x to 8x. Roughly how far does equity value fall?

    Show the worked solution

    Equity falls from Rs 2,400 crore to Rs 800 crore, a 66.7% drop on a 20% fall in EV. Debt is 7 x 800 = Rs 5,600 crore. At 10x, EV is Rs 8,000 crore; at 8x it is Rs 6,400 crore. Debt keeps its claim, so the equity slice absorbs the whole Rs 1,600 crore. At 7x leverage, equity moves 3.3 times as fast as EV, and a 7x multiple wipes it out.

    Why does equity fall so much further than the business?

    Think of a flat bought for Rs 1 crore with a Rs 70 lakh home loan. If the flat's price drops 20% to Rs 80 lakh, the bank still wants its Rs 70 lakh, so the owner's stake falls from Rs 30 lakh to Rs 10 lakh, a two-thirds loss. Debt is a fixed claim in rupees, so any change in enterprise value lands entirely on the equity that sits on top. Here debt is 7 x 800 = Rs 5,600 crore. At 10x, EV is Rs 8,000 crore and equity is Rs 2,400 crore, only 30% of the value.

    Debt stays at Rs 5,600 crore, so every rupee lost from EV comes out of equityDebt5,600Equity 2,400EV 8,00010x EBITDADebt5,600Equity 800EV 6,4008x EBITDADebt5,600Equity 0EV 5,6007x EBITDAEquity share of EV at 10x2,400 / 8,000 = 30%Multiplier = EV / equity8,000 / 2,400 = 3.33xMove from 10x to 8xEV-20.0%Equity-66.7%20% x 3.33 = 66.7%
    With debt fixed at Rs 5,600 crore, a fall in the multiple from 10x to 8x takes EV from Rs 8,000 crore to Rs 6,400 crore and equity from Rs 2,400 crore to Rs 800 crore, a 66.7% loss on a 20% fall in EV, and at 7x the equity is gone.
    The relationship
    ΔEE=ΔEVEV×EVE=−20%×8,0002,400=−66.7%\frac{\Delta E}{E} = \frac{\Delta EV}{EV} \times \frac{EV}{E} = -20\% \times \frac{8{,}000}{2{,}400} = -66.7\%
    Eequity value: EV less debt
    EVenterprise value: EBITDA times the multiple
    EV/Ethe leverage multiplier, how many rupees of business each rupee of equity carries
    What it says in wordsThe percentage move in equity is the percentage move in EV multiplied by EV over equity.

    So what does 7x leverage tell you about sensitivity?

    Leverage of 7x EBITDA means the debt alone is worth 7x EBITDA. Whenever the valuation multiple sits close to the leverage multiple, equity is a thin slice of the gap between them, and small changes in the multiple swing it hard. At 10x the gap is 3 turns; each turn of multiple is Rs 800 crore, a third of the equity. Software companies can carry this because their cash flows recur and their multiples are high, but the same 7x on a business valued at 8x leaves one turn of cushion. Say the limitation: the sum assumes debt is worth its face value, while in distress the debt itself would trade below par and soak up some of the loss.

    Where candidates lose it

    The common slip is answering 20%: the multiple fell 20%, so equity must too. That treats equity as if it were the whole business. Draw the stack, debt at the bottom in fixed rupees, and the answer is plain.

    The second loss is giving the number without the read. The interviewer asked what 7x tells you, and the answer is a sentence about equity being a thin, highly geared slice whose value is the gap between two multiples.

    What the interviewer asks next

    • At what EV/EBITDA multiple is the equity worth exactly zero?
    • If the company used Rs 800 crore of cash to repay debt first, how much would equity fall on the same de-rating?
    • How would a lender look at the same 7x number differently from an equity investor?

    Asked at Moelis & Company, Generalist, New York, 2026 (Wall Street Oasis): A tech company has leverage rate of 7X. What does it tell you about the about the impact on sensitivity?

  2. 032A machine turns a Rs 10 note into a Rs 20 note, but each use takes exactly one year, costs Rs 2 of electricity, and the machine breaks after 10 uses. At a 10% interest rate, what would you pay for it today?Compounding and time valueHardGoldman SachsDallas · 2026

    Try it first

    Before you discount anything: what is the machine's real net gain per use, measured at the end of the year?

    Show the worked solution

    About Rs 43. Each use ties up a Rs 10 note for a year, which at 10% costs you Rs 11 by year end, plus Rs 2 of electricity, and pays Rs 20. That is Rs 7 a year for ten years, and Rs 7 times the ten-year annuity factor of 6.1446 is Rs 43.01. If the electricity is paid at the start of each year the net is Rs 6.80 and the value Rs 41.78.

    What is one use of the machine actually worth?

    Think of lending a friend Rs 10 for a year and getting Rs 20 back. You doubled your note, but you also went a year without it, and that year had a price: whatever the money would have earned in the bank. The machine does not create Rs 10 a year; it creates Rs 20 at year end in exchange for Rs 10 today, and Rs 10 today is worth Rs 11 at year end at 10%. Take off Rs 2 of electricity and each use leaves Rs 7, received one year after you feed it the note.

    Ten uses, Rs 7 net each, discounted back to todayOne use, valued at year end:+20 out - 10 x 1.1 note in - 2 power = 7Sum of ten present valuesRs 43.01 (power paid up front: 41.78)6.36Yr 15.79Yr 25.26Yr 34.78Yr 44.35Yr 53.95Yr 63.59Yr 73.27Yr 82.97Yr 92.70Yr 10Dashed outline: Rs 7 received at the end of each year. Filled: what it is worth today at 10%.Forget the Rs 1 a year the note could have earned and you get Rs 49.16, too much.
    Each of the ten uses leaves Rs 7 at the end of its year once the Rs 11 cost of the note and the Rs 2 of electricity are taken off the Rs 20, and the ten present values fall from Rs 6.36 to Rs 2.70 and sum to Rs 43.01.
    The relationship
    PV=(20−10(1.1)−2)×1−1.1−100.10=7×6.1446=43.01PV = \big(20 - 10(1.1) - 2\big)\times\frac{1-1.1^{-10}}{0.10} = 7 \times 6.1446 = 43.01
    10(1.1)the Rs 10 note's value at year end had you kept it at 10%
    6.1446the present value of Rs 1 a year for ten years at 10%
    What it says in wordsFind the net gain of one use at the end of its year, then value ten of them as an annuity.

    Why does the timing of the electricity change the answer?

    The question does not say when you pay for power. If you pay at the end of the year, the net is the Rs 7 above. If you pay at the start, alongside the note, the Rs 2 also costs you a year of interest, Rs 2.20 by year end, and the net drops to Rs 6.80, worth Rs 41.78. State the timing assumption out loud, because an interviewer who wrote this puzzle is listening for whether you notice that cash flows need a date. The other common answer, Rs 49.16, discounts Rs 8 a year and is wrong for a reason, not a rounding: it treats the Rs 10 note as free to borrow.

    One check makes the answer believable. You could reproduce the machine with a bank loan: borrow Rs 10 at 10%, run the machine, repay Rs 11 and the Rs 2 of power, keep Rs 7. A buyer will pay up to the present value of that stream, and no more, because the bank can supply the money at 10% anyway.

    Where candidates lose it

    The usual answer is Rs 49.16: Rs 8 a year, discounted. It misses that the note fed in each year is capital with a cost. Candidates who think of each use as a project, with an outflow today and an inflow in a year, do not make this mistake.

    The second loss is giving one number with no assumption. Say when you assume the electricity is paid, give both values if asked, and the interviewer hears someone who dates every cash flow.

    What the interviewer asks next

    • What would you pay if the machine could be used once a year forever?
    • At what interest rate is the machine worth nothing?
    • If the machine could run two notes at once, what would it be worth?

    Asked at Goldman Sachs, Summer Analyst Interview, Dallas, 2026 (Wall Street Oasis): A mad scientist invents a machine that turns a standard $10 bill into a $20 bill

  3. 033Convert to percentages in your head, quickly: 3/32, 5/16 and 7/64.Mental mathsWarm upTreasuryCorporate FP&A

    Try it first

    Which of these is 7/64 as a percentage?

    Show the worked solution

    3/32 is 9.375%, 5/16 is 31.25% and 7/64 is 10.9375%. Anchor on 1/8 = 12.5% and halve: 1/16 is 6.25%, 1/32 is 3.125% and 1/64 is 1.5625%. Then multiply the rung by the numerator, or step from a nearby eighth: 7/64 is 1/8 less 1/64, which is 12.5 minus 1.5625.

    Why should powers of two be easy?

    Think of cutting a pizza: halve it, halve the halves, halve again, and you have eighths without measuring anything. Fractions with 2, 4, 8, 16, 32 or 64 at the bottom work the same way. Every power-of-two fraction is 12.5% halved some number of times, so you only need one anchor and the habit of halving. 1/8 is 12.5%, 1/16 is 6.25%, 1/32 is 3.125% and 1/64 is 1.5625%. The digits 125, 625, 3125, 15625 repeat in every problem of this kind.

    Start from 1/8 = 12.5% and halve; every answer is built from these rungs1/812.5%halve1/166.25%halve1/323.125%halve1/641.5625%Each rung is half the one above it3/323 x 3.125or 1/8 less 1/329.375%5/165 x 6.25or 1/4 plus 1/1631.25%7/6412.5 less 1.56258/64 less 1/6410.9375%
    Halving 12.5% gives 6.25%, 3.125% and 1.5625% for sixteenths, thirty-seconds and sixty-fourths, so 3/32 is three rungs of 3.125% or 9.375%, 5/16 is five rungs of 6.25% or 31.25%, and 7/64 is 12.5% less one rung of 1.5625%, or 10.9375%.

    When should you multiply up, and when should you step down from an eighth?

    Small numerators multiply cleanly: 3 x 3.125 is 9.375 and 5 x 6.25 is 31.25. When the numerator sits one below a multiple of the anchor, subtract instead: 7/64 is 8/64 less 1/64. That turns a seven-times table of 1.5625 into one subtraction, 12.5 less 1.5625, which is 10.9375%. Checking 3/32 the same way, 4/32 is 12.5%, less 3.125% is 9.375%, which matches. Two routes agreeing is the habit that catches a slipped digit.

    Where does this show up on a desk? Some bond prices are quoted in thirty-seconds of a point, so 3/32 of a point is 0.09375. Ownership splits and waterfall percentages often come in halves and quarters of halves. And an interviewer who asks this is really watching how fast you find a structure instead of reaching for longhand division.

    Where candidates lose it

    Candidates start long division, 3 divided by 32, and lose the decimal place or the tempo. The question is timed by feel: the interviewer wants an answer in a few seconds, and only the halving ladder gets you there.

    The second loss is rounding: 10.9% for 7/64 is fine as an estimate, but if the interviewer asked for the exact figure, it is 10.9375%, and the ladder gives it to you exactly.

    What the interviewer asks next

    • What is 11/64 as a percentage?
    • A bond is quoted at 98 and 5/32. What is that in decimal?
    • What is 1/128, and how do you get it from the ladder?
  4. 034A parent earns Rs 200 crore on its own and owns 60% of a subsidiary that earns Rs 50 crore and has book equity of Rs 300 crore. What are consolidated net income, net income attributable to the parent, and the non-controlling interest on the balance sheet?Accounting flow riddlesHardMoelis & CompanyNew York · 2025

    Try it first

    Consolidated net income, before anything is split off: which figure?

    Show the worked solution

    Consolidated net income is Rs 250 crore, Rs 230 crore is attributable to the parent, and non-controlling interest on the balance sheet is Rs 120 crore. Control brings in all of the subsidiary's Rs 50 crore profit. The outside holders' 40%, Rs 20 crore, is shown as profit attributable to non-controlling interest. On the balance sheet, their 40% of Rs 300 crore of book equity sits as a separate line in equity.

    Why take in all of a company you only partly own?

    Think of a family that controls a shop it co-owns with a cousin. The family runs it, banks its takings and pays its bills, so to describe what the family controls you count the whole shop, then note that a share of the profit belongs to the cousin. Control, not ownership share, decides consolidation, so the parent adds 100% of the subsidiary's revenue, costs, assets and debt, then shows the minority's share of profit and equity on separate lines. That is why consolidated net income is 200 plus 50, Rs 250 crore, and not 200 plus 30.

    Consolidation takes in 100% of the subsidiary, then hands 40% back on its own lineParentOwn profit Rs 200 croreSubsidiaryProfit 50, book equity 300owns 60%Outside shareholders own 40%Consolidated net income, Rs croreParent 200+50250Less non-controlling interest: 40% x 50-20Attributable to parent shareholders230Equity section, Rs croreParent's 60%: 180NCI 120Subsidiary book equity 300NCI on the balance sheet40% x 300 = 120
    The parent consolidates all Rs 50 crore of the subsidiary's profit to reach Rs 250 crore, peels off the outside holders' Rs 20 crore to leave Rs 230 crore for its own shareholders, and shows their 40% of Rs 300 crore of book equity, Rs 120 crore, as non-controlling interest.

    Where does non-controlling interest show up on each statement?

    On the income statement, net income of Rs 250 crore is split into Rs 230 crore for the parent's shareholders and Rs 20 crore for non-controlling interest. EPS uses the Rs 230 crore. On the balance sheet, non-controlling interest of Rs 120 crore sits inside total equity, next to the parent's own equity; if the subsidiary keeps its Rs 50 crore profit for the year, the line grows by Rs 20 crore. On the cash flow statement nothing is deducted, because the minority share of profit is not a cash payment; only dividends paid to the outside holders leave as cash, in financing. Non-controlling interest is a claim on the group by other shareholders, which is why an EV bridge adds it back alongside debt.

    State your assumption: the minority is measured at its share of the subsidiary's book equity, with no fair-value uplift or goodwill allocated to it on acquisition. Under the full goodwill method the line would be larger. And note the boundary case: had the parent owned 40% without control, it would use the equity method, report the same Rs 230 crore of net income, but show none of the subsidiary's revenue or debt.

    Where candidates lose it

    The common error is consolidating 60% of the subsidiary, getting Rs 230 crore and calling it consolidated net income. That is proportionate consolidation, which is not how control is accounted for. The Rs 230 crore is right, but it is the attributable figure, not the consolidated one.

    The second loss is computing NCI on the balance sheet as 40% of the profit, Rs 20 crore. Profit is a flow; the balance sheet line is a stock, 40% of the subsidiary's equity.

    What the interviewer asks next

    • Why does an EV bridge add non-controlling interest, and what goes wrong if you forget it?
    • If the parent bought the remaining 40% for Rs 150 crore, how would the accounts change?
    • How would the numbers look under the equity method at 40% ownership?

    Asked at Moelis & Company, Generalist, New York, 2025 (Wall Street Oasis): Minority interest on the 3 statements and debt waterfall

  5. 035A company trades at 2.0x book value. It has total assets of Rs 1,500 crore and a book debt-to-equity ratio of 0.5, with no other liabilities. What is its market capitalisation?Valuation and multiples riddlesCoreEvercoreSan Francisco · 2026

    Try it first

    What is book equity here?

    Show the worked solution

    Market capitalisation is Rs 2,000 crore. With no other liabilities, assets equal debt plus equity. A D/E of 0.5 means debt is half of equity, so assets are 1.5 times equity and book equity is Rs 1,500 crore over 1.5, which is Rs 1,000 crore, with Rs 500 crore of debt. At 2.0x book, the market values that equity at Rs 2,000 crore.

    How does a debt-to-equity ratio split the balance sheet?

    Think of a Rs 15 lakh car bought with a loan that is half the size of your down payment. If the loan is half the down payment, the car cost one and a half down payments, so the down payment was Rs 10 lakh and the loan Rs 5 lakh. A D/E of 0.5 does not mean half the assets are debt; it means debt is half of equity, so assets are 1.5 times equity. Rs 1,500 crore over 1.5 gives book equity of Rs 1,000 crore and debt of Rs 500 crore.

    D/E splits the assets; P/B turns book equity into market valueAssets1,500Equity 1,000Debt 500= 1,500x 2.0Market cap2,000Book assetsFunded byMarket valueD/E = 0.5, so D = 0.5 ED + E = 1.5 E = 1,500E = 1,500 / 1.5 = 1,000Market cap = 2.0 x 1,000= Rs 2,000 croreWrong: equity = half of assets750 x 2.0 = 1,500D/E is debt over equity, not over assets
    A D/E of 0.5 splits Rs 1,500 crore of assets into Rs 1,000 crore of equity and Rs 500 crore of debt, and at 2.0x book the market values the equity at Rs 2,000 crore, not the Rs 1,500 crore you get by treating half the assets as equity.
    The relationship
    Market cap=P/B×Assets1+D/E=2.0×1,5001.5=2,000\text{Market cap} = \text{P/B} \times \frac{\text{Assets}}{1 + D/E} = 2.0 \times \frac{1{,}500}{1.5} = 2{,}000
    P/Bprice to book: market value of equity over book equity
    D/Ebook debt over book equity
    1 + D/Eassets as a multiple of equity when there are no other liabilities
    What it says in wordsDivide assets by one plus the debt-to-equity ratio to get book equity, then apply the price-to-book multiple.

    What assumptions sit under the answer, and what could you add?

    The question rules out other liabilities, such as payables and provisions, and that matters: a real balance sheet has plenty, and then assets are not just debt plus equity. Say the identity you are using, assets equal liabilities plus equity, before you lean on it. You can also go one step further than asked. With Rs 500 crore of debt and no cash given, enterprise value is about Rs 2,500 crore. And a P/B of 2.0 says the market thinks the assets earn more than their cost of capital; read it next to return on equity rather than on its own.

    Where candidates lose it

    The slip is reading D/E as debt over assets: half of Rs 1,500 crore is Rs 750 crore of equity, doubled to Rs 1,500 crore. Candidates do it because 0.5 sounds like half. Write D = 0.5E and the algebra decides for you.

    The second loss is applying P/B to total assets. Price to book compares market value with book equity only, so the multiple must land on the Rs 1,000 crore, not on the Rs 1,500 crore.

    What the interviewer asks next

    • If the company also had Rs 300 crore of payables, what would market cap be?
    • What return on equity would justify a P/B of 2.0 if the cost of equity is 12% and growth is zero?
    • With Rs 200 crore of cash, what is enterprise value?

    Asked at Evercore, Mergers and Acquisitions, San Francisco, 2026 (Wall Street Oasis): Given P/B, total assets, and D/E ratio calculate Market Cap

  6. 036A business has a 10% operating margin. Next year revenue grows 20% and costs grow 15%. What is the new margin?Percentages and averagesCoreCorporate FP&ABusiness finance

    Try it first

    Quick call: where does the margin land?

    Show the worked solution

    The new margin is 13.75%. Take revenue of 100, so cost is 90 and profit 10. Revenue grows to 120 and cost to 90 x 1.15 = 103.5, leaving profit of 16.5. The margin is 16.5 over 120, which is 13.75%. Profit itself grows 65%, because a 5 point gap between growth rates is large next to a 10% margin.

    Why is the answer not simply 15%?

    Think of a tea stall that takes in Rs 100 a day and spends Rs 90. If takings rise 20% and spending rises 15%, the extra takings are Rs 20 but the extra spending is Rs 13.50, because 15% is taken on 90, not on 100. Growth rates act on their own bases, so a margin moves by the gap in rupees, not by the gap in percentage points. Work in rupees on a base of 100 and the trap disappears: profit goes from 10 to 16.5 on revenue of 120.

    Revenue adds 20 on a base of 100; cost takes back only 13.5 on a base of 9010Old profit10% of 100+20Revenue+20% x 100-13.5Cost+15% x 9016.5New profiton 120Revenue100 to 120+20%Cost90 to 103.5+15%Profit10 to 16.5+65%Margin16.5 / 12013.75%Not 10% + (20% - 15%) = 15%the two growth rates act on different bases
    On revenue of 100, profit of 10 gains 20 from revenue growth and loses 13.5 to cost growth, ending at 16.5 on revenue of 120, a margin of 13.75% rather than the 15% you get by adding the growth gap to the margin.
    The relationship
    m1=1−(1−m0)1+gc1+gr=1−0.90×1.151.20=13.75%m_1 = 1 - (1-m_0)\frac{1+g_c}{1+g_r} = 1 - 0.90 \times \frac{1.15}{1.20} = 13.75\%
    m0, m1the old and new margins
    g_r, g_crevenue growth and cost growth
    1 - m0cost as a share of revenue before the change
    What it says in wordsThe new cost share is the old cost share scaled by cost growth over revenue growth, and the margin is what is left.

    Why does a thin margin make the move so large?

    Profit is the small gap between two big numbers, so a modest difference in their growth rates is a big change to the gap. The thinner the starting margin, the more operating leverage a growth gap carries: here profit grows 65% on 20% revenue growth. Run the same 20% and 15% from a 30% margin and the new margin is 32.9%, a smaller relative move, because cost is a smaller share of revenue. Say the limit too: the formula assumes all cost grows at 15%. In practice fixed costs grow slowly and variable costs track volume, so you would split the cost line before trusting the answer.

    Where candidates lose it

    The fast wrong answer is 15%: 10% plus the 5 point gap between the growth rates. It treats both rates as if they applied to the same base, when cost growth applies to 90 and revenue growth to 100.

    The second loss is stopping at the margin. The interviewer often wants the profit growth too, and 65% on 20% revenue growth is the sentence that shows you understand operating leverage.

    What the interviewer asks next

    • What cost growth would keep the margin at exactly 10%?
    • If half the cost base is fixed and does not grow, what is the new margin?
    • Revenue falls 10% and costs fall 5%. What happens to a 10% margin?
  7. 037Estimate how many new narrow-body aircraft India's airlines will need each year over the next decade.Estimation and market sizingHardRothschild & CoParis · 2026Rothschild & CoParis · 2026

    Try it first

    Which split gives you the cleanest structure for this estimate?

    Show the worked solution

    About 106 a year, on stated assumptions. Assume 15 crore domestic trips a year. A 180-seat jet, 85% full, flying 5 sectors a day for 350 days carries about 2.68 lakh passengers, so the fleet is about 560 jets. At 8% traffic growth it needs 1,209 in ten years, 65 more a year, plus about 41 retirements a year on a 20-year life.

    How do you turn passengers into aircraft?

    Think of a school deciding how many buses to buy. It counts the children who need a seat, divides by how many one bus can carry in a day, then adds buses for next year's bigger intake and for the old ones being scrapped. Aircraft demand is the same: fleet size is passenger trips divided by what one jet carries in a year, and new orders are fleet growth plus replacement. State each number as an assumption, because the interviewer cares about the structure first and the inputs second.

    Start with traffic. Assume about 15 crore domestic passenger trips a year, and tell the interviewer you would check the current figure in the aviation regulator's monthly traffic data. A narrow-body has about 180 seats; assume 85% of them are filled, 5 sectors a day and 350 flying days. That is 267,750 passengers a jet a year, so today's fleet is about 560 jets.

    Split demand into growth and replacement, then size each from passengersNew narrow-bodies a yearabout 106Growth: fleet added(1,209 - 560) / 10 = 65Replacement: retirements812 avg fleet / 20 yrs = 41Fleet needed today15 crore / 2.68 lakh = 560Fleet in ten years560 x 1.08^10 = 1,209Passengers per jet a year180 x 85% x 5 x 350= 267,750: the assumption that moves the answer mostAverage fleet over the decadeRetire 1 jet in 20 each year(young fleets retire fewer)Left out: international short-haul,spare jets, groundings
    Fifteen crore trips at about 2.68 lakh passengers per jet need about 560 jets today and 1,209 in ten years at 8% growth, which adds 65 jets a year, and replacing jets on a 20-year life adds about 41 more, for roughly 106 new narrow-bodies a year.

    Which assumption moves the answer most, and what did you leave out?

    Growth carries more of the answer than replacement: at 8% a year the fleet more than doubles in ten years, adding 65 jets a year, while retirements on a 20-year life add 41. The number most worth defending is passengers per jet, because a sixth sector a day would cut the fleet by a sixth and every later number with it. Then name what you left out: international short-haul routes also fly narrow-bodies, airlines hold spare aircraft for maintenance, and engine problems can ground jets for months. Each pushes the true need above 106. Also say that orders and deliveries differ: airlines order years ahead, so order books can be far larger than a decade's need.

    Where candidates lose it

    Candidates jump to a number they half remember from a news story about a record order. That is not an estimate, and it is often an order book spread over many years, not annual demand. Build it from passengers and the interviewer can follow every step.

    The second loss is forgetting replacement entirely, or adding it as a fraction of today's fleet instead of the growing one. Average the fleet over the decade, or at least say that retirements grow with it.

    What the interviewer asks next

    • How does the answer change if load factors rise to 90%?
    • What share of the demand would one airline with a third of the market need?
    • How would you size wide-body demand differently?

    Asked at Rothschild & Co, Asset Management, Paris, 2026 (Wall Street Oasis): Can You estimate number of flights solds by airbus
    Asked at Rothschild & Co, Asset Management, Paris, 2026 (Wall Street Oasis): first part was more about market sizing and logic reasoning

  8. 038You have a 3-litre bottle, a 4-litre bottle and a tap. Neither bottle has markings. How do you measure exactly 2 litres?Logic and counting brainteasersWarm upNomuraNew York · 2026

    Try it first

    What is the fewest number of fills and pours that leaves exactly 2 litres?

    Show the worked solution

    Fill the 3-litre bottle, pour it into the 4, fill the 3 again and pour until the 4 is full: 2 litres are left in the 3-litre bottle. The 4 already holds 3 litres, so it takes only 1 more. That is four moves. A longer route fills the 4 first and ends with 2 litres in the 4-litre bottle after six moves.

    How do you avoid getting lost halfway through?

    Think of following a recipe that says pour half the milk into the other pan: if you do not write down how much is in each pan, two steps later you have no idea. Track the pair of levels after every single move, written as (3-litre, 4-litre), and the puzzle becomes a short walk through a few states instead of a juggling act. There are only three kinds of move: fill a bottle from the tap, empty a bottle, or pour one into the other until the source is empty or the target is full.

    Write down both bottles after every move; 2 litres appears on move four3 L04 L0Start3 L34 L01. Fill the 33 L04 L32. Pour 3 into 43 L34 L33. Fill the 33 L24 L44. Top up the 4Longer route, six moves: fill 4, pour into 3 (1 left), empty 3, pour the 1 across,fill 4, top up the 3 (needs 2): 2 litres left in the 4 litre bottle.
    Filling the 3-litre bottle, pouring it into the 4, filling the 3 again and topping up the 4 leaves exactly 2 litres in the 3-litre bottle after four moves, because the 4-litre bottle had room for only 1 more litre.

    Say the states out loud as you go: (0, 0), (3, 0), (0, 3), (3, 3), (2, 4). The interviewer can check each one in a second, and if you slip, you can see where.

    Which amounts can these two bottles measure at all?

    Every pour moves water in steps of 3 and 4, so the amounts you can isolate are the combinations of 3 and 4 with whole numbers in front: 4 minus 3 is 1, 3 times 2 minus 4 is 2, and so on. Because 3 and 4 share no common factor bigger than 1, these bottles can measure every whole number of litres from 1 to 4, and up to 7 if you count both bottles together. With a 4-litre and a 6-litre bottle you could never get an odd number, since every amount would be a multiple of 2. Saying that rule turns a party trick into reasoning.

    Where candidates lose it

    Candidates start pouring in their head without writing states and lose track after three moves, then restart. It looks like panic even when it is not. Say the pair of numbers after each move and the interviewer follows you.

    The second loss is stopping at a long route without checking for a short one. Mentioning that you found 2 litres in four moves, and that a six-move route also works, shows you looked for the efficient answer.

    What the interviewer asks next

    • With the same bottles, how do you measure exactly 1 litre?
    • With a 6-litre and a 9-litre bottle, can you measure 4 litres?
    • How would you prove that a given amount cannot be measured?

    Asked at Nomura, Equity Capital Markets, New York, 2026 (Wall Street Oasis): How much water can you fill using 1 3liter and 1 4liter bottle using each other?

  9. 0391% of a company's invoices are fraudulent. A screening rule flags 90% of fraudulent invoices and 5% of clean ones. If an invoice is flagged, how likely is it to be fraud?Probability and expected valueCoreWolverine TradingChicago · 2017

    Try it first

    A flagged invoice: what is the chance it is really fraud?

    Show the worked solution

    About 15.4%. Picture 10,000 invoices. 100 are fraudulent and the screen flags 90 of them. 9,900 are clean and the screen flags 5% of them, 495. The flagged pile holds 585 invoices, of which 90 are fraud, so a flag means fraud only 90 times in 585. The rare base rate swamps the screen's accuracy.

    Why is a 90% accurate screen right only 15% of the time?

    Think of a smoke alarm that never misses a fire but also goes off whenever someone makes toast. In a house where fires are rare and toast is daily, almost every alarm is toast. When the thing you are hunting is rare, even a small false positive rate on the large innocent pile produces more false alarms than true hits. Here 5% of 9,900 clean invoices is 495 false flags, against only 90 true ones.

    Count 10,000 invoices through the screen, then look only at the flagged pileAll invoices10,000Fraud, 1%100Clean, 99%9,900Flagged, 90%90Missed10Flagged, 5%495Passed9,405Flagged pile: 585495 clean90 fraud90 / 58515.4%of flags arereal fraud
    Of 10,000 invoices, the screen flags 90 of the 100 frauds and 495 of the 9,900 clean invoices, so the flagged pile of 585 is only 15.4% fraud even though the screen catches 90% of frauds.
    The relationship
    P(F∣flag)=0.90×0.010.90×0.01+0.05×0.99=0.0090.0585=15.4%P(F\mid \text{flag}) = \frac{0.90 \times 0.01}{0.90 \times 0.01 + 0.05 \times 0.99} = \frac{0.009}{0.0585} = 15.4\%
    P(F | flag)the chance an invoice is fraud given that it was flagged
    0.01the base rate of fraud
    0.05the false positive rate on clean invoices
    What it says in wordsTrue flags divided by all flags, where all flags include the false ones from the much larger clean pile.

    What would make the screen useful, and how would an auditor use it?

    Cutting the false positive rate does far more than raising the catch rate. At a 0.5% false positive rate the flagged pile would be 90 frauds and about 49.5 clean invoices, and a flag would mean fraud 65% of the time. Raising the catch rate from 90% to 100% would only move the answer from 15.4% to about 16.8%. In practice a screen like this is a triage tool: it shrinks 10,000 invoices to 585 for a human to review, and that review is where the 495 false alarms are cleared. Say the limit too: the 1% base rate is itself an estimate, and the answer is only as good as it is.

    Where candidates lose it

    The trap answer is 90%, which swaps the chance of a flag given fraud for the chance of fraud given a flag. Interviewers ask this question to see whether you notice the swap.

    Work in counts, not formulas. Saying "imagine 10,000 invoices" makes every number concrete, and the 495 false alarms jump out before you have written a single probability.

    What the interviewer asks next

    • If an invoice is flagged twice by two independent screens, what is the chance it is fraud?
    • What false positive rate would make a flag mean fraud at least half the time?
    • How would the answer change if fraud were 10% of invoices?

    Asked at Wolverine Trading, Prop Trading, Chicago, 2017 (Wall Street Oasis): Phone interviews were pretty standard brainteasers and fit questions. There was a Bayes question

  10. 040Receivables are Rs 300 crore at quarter end and revenue for the quarter was Rs 450 crore. A junior analyst reports days sales outstanding of 243 days. What went wrong, and what is the right figure?Working capital and cash riddlesWarm upCorporate FP&ABig Four

    Try it first

    What is the right DSO?

    Show the worked solution

    The right DSO is about 61 days; the junior set a quarter's revenue against a full year of days. DSO is receivables over revenue times the days in the same period. Either annualise revenue, 450 x 4 = 1,800, giving 300 / 1,800 x 365 = 60.8 days, or keep the quarter and multiply by its 91.25 days. The 243 is exactly four times too large.

    What does DSO actually measure?

    Think of a tailor who sells Rs 1,000 of clothes a day and is owed Rs 30,000 by customers. Customers are, on average, 30 days behind. DSO is receivables divided by revenue per day, so the revenue and the day count must come from the same period. Rs 450 crore over a quarter is about Rs 4.93 crore a day, and Rs 300 crore of receivables is about 61 days of that.

    The error: a quarter's revenue set against a year's daysQ3 working capital note (draft)Trade receivables, quarter endRs 300 croreRevenue, three monthsRs 450 croreDSO = 300 / 450 x 365 = 243 dayswrongDSO = 300 / (450 x 4) x 365 = 61 daysor 300 / 450 x 91.25 days = 61 daysAnnualise the revenue, or use the days in the quarter.Never mix the two.Why it slipped throughThe formula is right; theperiods are not. 450 is threemonths of sales, 365 is twelvemonths of days.The ratio comes out exactlyfour times too large: 243 is61 x 4.Check: 243 days means acustomer pays eight monthslate. Ask if that is plausible.
    Dividing receivables of Rs 300 crore by one quarter's revenue of Rs 450 crore and multiplying by 365 days gives 243 days, exactly four times the right answer of about 61 days, which comes from annualising revenue or using the 91 days in the quarter.
    The relationship
    DSO=ReceivablesRevenue in period×days in period=300450×91.25=60.8\text{DSO} = \frac{\text{Receivables}}{\text{Revenue in period}} \times \text{days in period} = \frac{300}{450} \times 91.25 = 60.8
    Revenue in periodsales over the same span of time as the day count
    days in period365 for a year, about 91 for a quarter
    What it says in wordsMatch the revenue period to the day count, and DSO is the number of days of sales still unpaid.

    How would you have caught 243 days before it went out?

    Run a plausibility check before any number leaves your desk. 243 days says the average customer pays eight months after the sale, which almost no ordinary business tolerates, so the number should have triggered a second look on sight. A second check: compare with last quarter's DSO computed the same way. A sudden fourfold jump is almost always a formula or period error, not a collections crisis. Say the limitation of the corrected figure too: quarter-end receivables against average daily sales can swing with seasonality, so analysts often use average receivables over the period, and a strong last month of the quarter will push DSO up even when customers pay on time.

    Where candidates lose it

    The error itself is the trap: quarterly revenue with 365 days. It happens most when a template built for annual numbers is fed quarterly data. Spot the factor of four and say so.

    The second loss is fixing it silently. The interviewer wants you to name the rule, match the period of revenue to the days you multiply by, and to say how you would catch it next time.

    What the interviewer asks next

    • If the last month of the quarter carried half the quarter's sales, how would that distort DSO?
    • How would you compute days payable outstanding from quarterly data?
    • Why might an analyst prefer average receivables to the quarter-end figure?
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