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Debt Capital Markets puzzles, solved step by step

Puzzles
100
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16
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13
Hard
30
Topic
All topicsLeverage, coverage and cash flow9Mental maths and numeracy8Estimation and market sizing7Logic and brainteasers8Cost of capital and valuation riddles7Bond pricing and yield7Compounding, PIK and fees6Issuance and refinancing arithmetic8Credit spreads and default probability8Duration and convexity8Capital structure and recovery8Probability and expected value10Yield curve and forward rates6
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Showing 91–100 of 100
  1. 091A fund wants to buy Rs 5,000 crore of a bond segment where average daily trading is Rs 1,000 crore, and it will not take more than 2% of each day's volume. How long does it take to build the position, and what does that say about liquidity?Estimation and market sizingCoreFixed income asset managementSyndicate desks

    Try it first

    Roughly how long does the fund need?

    Show the worked solution

    About 250 trading days, roughly a year. The fund may buy 2% of Rs 1,000 crore, which is Rs 20 crore a day, and Rs 5,000 crore divided by Rs 20 crore is 250. The position is five full days of the whole market's trading, which makes it illiquid for this fund: it would take about as long to sell as to buy.

    Why is the answer not five days?

    Imagine wanting to buy 500 kilos of tomatoes from a village market that sells 100 kilos a day. In theory it is five days of the market's supply; in practice, if you take everything, prices jump and the regular buyers go home empty handed. So you decide to take no more than a couple of kilos a day. The limit on your share of daily volume, not the market's total volume, sets how fast you can trade, and here that limit is Rs 20 crore a day. Rs 5,000 crore at Rs 20 crore a day is 250 days.

    Size divided by what you may trade each day is the time it takes2% of Rs 1,000 crore a dayRs 20 crore a day250 days10% of Rs 1,000 crore a dayRs 100 crore a day50 days2% of Rs 10,000 crore a dayRs 200 crore a day25 days0a quarterhalf a yearabout a trading year
    At 2% of Rs 1,000 crore a day the fund buys Rs 20 crore daily and needs 250 trading days, about a year, while a 10% share of volume would take 50 days and a segment ten times as liquid 25 days.
    The relationship
    Days=PositionADV×participation=5,0001,000×2%=250\text{Days} = \frac{\text{Position}}{\text{ADV} \times \text{participation}} = \frac{5{,}000}{1{,}000 \times 2\%} = 250
    Positionthe amount to buy, Rs 5,000 crore
    ADVaverage daily volume, Rs 1,000 crore
    participationthe most of each day's volume the fund will take, 2%
    What it says in wordsDivide what you want to trade by what you are allowed to trade each day.

    What does this tell you about liquidity?

    Liquidity belongs to a position, not to a market: the same bonds are liquid for a Rs 50 crore holder and illiquid for a Rs 5,000 crore holder. Days to trade is the simplest honest measure. At 250 days, the fund carries a year of market risk while it builds, and faces the same year if it ever needs to sell, which matters most exactly when others are selling too. Daily volume also falls in a stressed market, so the exit could take longer than the entry. A desk would size the position down, accept a higher participation rate and more price impact, or look for a block trade or a new issue where it can take size in one go.

    Where candidates lose it

    The fast wrong answer is five days, dividing the position by the whole market's volume. It ignores the participation limit that the question hands you.

    The second loss is giving 250 days and stopping. The interviewer wants the implication: this position is illiquid for this fund, the exit is the real risk, and there are ways to get size faster, each with a cost.

    What the interviewer asks next

    • If the fund accepts 5% of daily volume, how long does it take, and what does it give up?
    • Daily volume halves in a sell-off. How long would it take to exit the full position at 2%?
    • Why might a new issue be a better way to build this position than the secondary market?
  2. 092A high yield bond portfolio earns a spread of 450 basis points and an investment grade portfolio earns 120. Recovery on high yield defaults is 30%, and investment grade defaults are negligible. What annual default rate can the high yield portfolio absorb before it earns less than investment grade?Credit spreads and default probabilityCoreCredit researchFixed income asset management

    Try it first

    Roughly what default rate wipes out the extra spread?

    Show the worked solution

    About 4.71% of the portfolio defaulting each year. High yield earns 330 basis points more than investment grade. Each default costs 70% of face, since 30% is recovered, so each 1% of defaults costs 70 basis points. 330 divided by 70 is 4.71%. Below that default rate high yield still earns more; above it, less.

    Why divide by the loss, not the default rate?

    A shop that sells on credit loses less than the full bill when a customer fails to pay, if it can take back some of the goods. Bonds work the same way. The annual cost of defaults is the default rate times the loss given defaultThe share of face value lost when a borrower defaults, which is one minus the recovery rate., so the extra spread covers defaults up to the extra spread divided by the loss given default. With 30% recovered, each default loses 70%, and 330 basis points of extra spread covers 4.71% a year.

    Excess spread buys a measurable cushion of defaults0%1%2%3%4%5%6%7%8%0200400Annual default rate in the high yield portfoliobp a yearextra spread over IG: 330 bpcredit loss = default rate x 70break-even 4.71%HY aheadHY behind
    Annual credit loss rises by 70 basis points for each 1% of defaults and crosses the 330 basis points of extra spread at 4.71%, so high yield earns more than investment grade only while defaults stay below that rate.
    The relationship
    d∗=sHY−sIG1−R=450−1200.70=471 bp=4.71%d^{*} = \frac{s_{HY} - s_{IG}}{1 - R} = \frac{450 - 120}{0.70} = 471\text{ bp} = 4.71\%
    s_HY, s_IGthe two portfolios' spreads, in basis points a year
    Rrecovery on high yield defaults, 30%
    d*the break-even annual default rate
    What it says in wordsThe default rate that exactly uses up the extra spread is that spread divided by the loss on each default.

    What does the number leave out?

    A break-even default rate is a cushion, not a forecast: whether 4.7% is comfortable depends on how often default rates have run above it and for how long. It also assumes investment grade loses nothing; if investment grade lost 10 basis points a year, the break-even would rise to 4.86%. Recovery is the soft number: it tends to fall in exactly the years defaults rise, so a 30% assumption can flatter the cushion. And the calculation ignores price volatility, which matters to anyone who must mark the portfolio before the bonds mature.

    Where candidates lose it

    The frequent slip is saying 3.3%, treating every default as a total loss. With 30% recovery, each default costs 70%, and dividing by 0.7 is the whole point of the puzzle.

    The second is using the full 450 basis points rather than the 330 excess over investment grade. The question compares two portfolios, so only the difference in spread buys the cushion.

    What the interviewer asks next

    • If recovery falls to 20% in a downturn, what is the new break-even default rate?
    • What default rate does the full 450 basis point spread cover if investors demanded no extra return for risk?
    • Why do recoveries tend to fall when default rates rise?
  3. 093A Rs 1,000 crore fund charges a 2% management fee and a 20% performance fee on returns above fees, and earns a 10% gross return. Investors push the management fee down to 1.5%. What performance fee rate keeps the manager's revenue the same?Compounding, PIK and feesCoreTwo SigmaNew York · 2026

    Try it first

    Roughly what performance fee replaces the lost 0.5%?

    Show the worked solution

    A performance fee of about 24.7%. Today the manager earns Rs 20 crore of management fee plus 20% of the Rs 80 crore return after fees, Rs 16 crore, for Rs 36 crore. At 1.5% the fixed fee is Rs 15 crore and the return after fees rises to Rs 85 crore, so the performance fee must bring Rs 21 crore: 21 / 85 = 24.7%. The swap is exact only at a 10% gross return.

    Why does the performance fee base change?

    Think of a salesperson paid a fixed salary plus a share of profit after salary. Cut the salary and the profit after salary goes up, so the share needed to make up the difference is smaller than a straight swap would suggest. The performance fee is charged on the return left after the management fee, so lowering the management fee enlarges the base the performance fee is charged on. That is why the answer is 24.7%, not 25%.

    Same revenue at 10%, different revenue anywhere elsefixed 20perf 16362% and 20%fixed 15perf 21361.5% and 24.7%5% gross10% gross15% grossold 26.0new 23.6new 48.4old 46.0equal at 36Revenue, Rs crore, by gross return
    At a 10% gross return both fee terms pay the manager Rs 36 crore, but at 5% the old terms pay 26.0 against 23.6 and at 15% the new terms pay 48.4 against 46.0, so the trade shifts revenue from weak years to strong ones.
    The relationship
    15+x (100−15)=20+0.20 (100−20)  ⇒  x=36−1585=24.7%15 + x\,(100 - 15) = 20 + 0.20\,(100 - 20) \;\Rightarrow\; x = \frac{36 - 15}{85} = 24.7\%
    100the 10% gross return on Rs 1,000 crore, in Rs crore
    15, 20the new and old management fees, Rs crore
    xthe new performance fee rate
    What it says in wordsSet the new fixed fee plus the new share of the bigger base equal to today's total.

    Is the manager really indifferent?

    Only at a 10% gross return. At 5% gross, the old terms pay Rs 26.0 crore and the new pay Rs 23.6 crore; at 15%, the old pay Rs 46.0 crore and the new Rs 48.4 crore. Swapping fixed fee for performance fee moves the manager's revenue out of weak years and into strong ones, so the trade depends on the return you assume. Investors who push for it pay less when things go badly and more when they go well, which is often exactly what they want. A real fund would also have a hurdle rate, a high-water mark and fees charged on average rather than opening assets; each changes the arithmetic, not the logic.

    Where candidates lose it

    The common answer is 25% or 22.5%, from treating the two fees as if they sat on the same base. The management fee is on assets and the performance fee is on return after fees, so moving one changes the base of the other.

    The second loss is saying the new terms are equivalent. They are equal at one assumed return only; say so, and give the 5% and 15% cases to show which way the risk moved.

    What the interviewer asks next

    • What performance fee keeps revenue the same if the assumed gross return is 15%?
    • The fund adds a 5% hurdle, with performance fee only on returns above it. How does that change the answer at 10%?
    • From the investor's side, which fee terms would you prefer in a year you expect to be weak, and why?

    Asked at Two Sigma, Equity Capital Markets, New York, 2026 (Wall Street Oasis): the 2/20 rule, and if one part of this equation changed, how would the other variable make up for it

  4. 094A borrower can pay 11% in cash or 12% as payment-in-kind on a Rs 500 crore five-year loan. Any cash it does not pay out can be reinvested in the business at a 15% return. Which option leaves the equity better off after five years, and when would the answer flip?Compounding, PIK and feesHardLeveraged financePrivate credit

    Try it first

    The business earns 15%, more than the 12% PIK rate. Does PIK win?

    Show the worked solution

    Paying cash leaves the equity about Rs 10 crore better off. PIK debt compounds at 12% to Rs 881 crore. The Rs 55 crore a year that PIK keeps in the business, reinvested at 15%, grows to only Rs 371 crore, so net debt is Rs 510 crore against Rs 500 crore with cash pay. The answer flips if the business earns more than about 16.4% on the cash, or if paying cash would starve it.

    What exactly is being compared?

    Deferring a credit card bill and investing the money instead only pays if your investment grows faster than the card's balance. PIKPayment in kind: interest is added to the loan balance instead of being paid in cash, so the debt compounds. interest is that deferral, made formal. Under PIK the whole loan compounds at the PIK rate, while only the cash you did not pay compounds at your own return, so the two piles grow on different bases. Line up where each option leaves the company at year 5: debt owed, less any cash built up by reinvesting what was not paid.

    PIK saves cash today, but the debt compounds faster than the cash doesCash pay at 11%500Debt owed0Cash kept500Net debtPIK at 12%, cash reinvested at 15%881Debt owed371Cash kept510Net debtPIK leaves net debt 10 crore higher at 15%. It wins only if the cash earns more than 16.4% a year.
    Paying 11% in cash leaves Rs 500 crore of debt, while PIK at 12% grows the debt to Rs 881 crore against Rs 371 crore of reinvested cash, so PIK ends Rs 10 crore worse unless the cash earns above 16.4%.
    The relationship
    500(1.12)5=881.255⋅1.155−10.15=370.8881.2−370.8=510.3500(1.12)^5 = 881.2 \qquad 55\cdot\frac{1.15^5 - 1}{0.15} = 370.8 \qquad 881.2 - 370.8 = 510.3
    500(1.12)^5the PIK balance after five years
    55the 11% cash interest that PIK lets the company keep each year
    370.8that cash reinvested at 15% for five years
    What it says in wordsNet debt under PIK is the compounded loan less the compounded cash that was kept.

    When does the answer flip?

    Solve for the return that makes the kept cash worth exactly the extra Rs 381 crore of PIK debt: about 16.4%. PIK is cheap only if the cash it saves earns more than the PIK rate, and here more still, because the PIK rate carries a one point premium over cash pay. At a 12% reinvestment return the kept cash reaches only Rs 349 crore. Two further reasons can flip the answer in practice: a borrower whose cash is scarce may not be able to pay 11% without cutting investment it needs, and a borrower expecting to refinance or be sold early carries the PIK compounding for less time. Tax treatment of PIK interest can also differ; confirm it rather than assuming.

    Where candidates lose it

    The trap is comparing 15% with 12% and declaring PIK the winner. The PIK rate applies to the whole Rs 500 crore; the 15% applies only to Rs 55 crore a year. Rates on different bases cannot be compared directly.

    The second loss is ignoring the one point PIK premium. It is why the break-even is near 16%, not 12%, and it is the number a leveraged finance desk would negotiate hardest.

    What the interviewer asks next

    • If both options were priced at 11%, what reinvestment return would make them equal?
    • The company is sold at the end of year 3. Does that help or hurt the PIK option?
    • Why would a lender accept PIK at all, and what does it ask for in return?
  5. 095A company has EBITDA of Rs 300 crore and Rs 1,000 crore of debt, all floating at 10%. The benchmark rate rises by 150 basis points. What happens to its interest cover?Leverage, coverage and cash flowWarm upCorporate bankingLeveraged finance

    Try it first

    Where does interest cover land?

    Show the worked solution

    Interest cover falls from 3.0x to about 2.61x. All the debt is floating, so the rate goes from 10% to 11.5% and interest from Rs 100 crore to Rs 115 crore. EBITDA is unchanged at Rs 300 crore, so cover is 300 / 115 = 2.61x. A 15% rise in the interest bill cuts cover by about 13%, with no change in the business at all.

    Why does a small rate move matter so much?

    A household on a floating home loan feels a rate rise in the very next EMI, even though nothing about its salary has changed. Companies with floating debt feel it the same way. Floating rate debt passes a benchmark rise straight into the interest bill, so coverage falls even when the business is doing exactly as well as before. Here 150 basis points on Rs 1,000 crore is Rs 15 crore a year, a 15% rise in interest.

    Floating debt passes a rate rise straight into coverageInterest, Rs crore100Rate 10.0%115Rate 11.5%Interest cover, EBITDA / interest3.00xBefore2.61xAfter +150 bp2.0xEBITDA stays at Rs 300 crore; only the interest line moves
    A 150 basis point rise lifts interest on Rs 1,000 crore of floating debt from Rs 100 crore to Rs 115 crore, and with EBITDA unchanged at Rs 300 crore, interest cover falls from 3.0x to 2.61x.
    The relationship
    Cover=EBITDAD×r:3001,000×10%=3.0×  →  3001,000×11.5%=2.61×\text{Cover} = \frac{EBITDA}{D \times r}: \quad \frac{300}{1{,}000 \times 10\%} = 3.0\times \;\to\; \frac{300}{1{,}000 \times 11.5\%} = 2.61\times
    EBITDAearnings before interest, tax, depreciation and amortisation, Rs 300 crore
    Dfloating rate debt, Rs 1,000 crore
    rthe all-in floating rate, 10% before and 11.5% after
    What it says in wordsCoverage is earnings over the interest bill, and on floating debt the bill moves with the benchmark.

    What would a lender ask next?

    Two questions: how much headroom is left, and how much is hedged. Cover would reach 2.0x only if interest rose to Rs 150 crore, a rate of 15%, so the benchmark would need to climb another 350 basis points from here. Hedging changes the answer more than anything else: if 60% of the debt were swapped to fixed, the same rise would add only Rs 6 crore of interest and cover would fall only to 2.83x. A banker who sees all-floating debt at 3.0x cover asks about hedging before anything else.

    Where candidates lose it

    The common slip is saying cover is unchanged because EBITDA did not move. The ratio has two sides, and floating debt makes the bottom one move.

    The second is reporting 2.6x without the cause: interest up 15%, from Rs 100 crore to Rs 115 crore. The interviewer wants to hear that the business is unchanged and the capital structure did the damage.

    What the interviewer asks next

    • If covenants require cover of at least 2.5x, how far can the benchmark rise before the breach?
    • What share of the debt would need to be fixed to keep cover above 2.8x after the rise?
    • Why might EBITDA also fall when rates rise, and what does that do to the answer?
  6. 096Two Rs 1,000 crore loans are sold in the broadly syndicated market. A term loan A amortises 20% a year over 5 years. A term loan B amortises 1% a year and repays the rest as a bullet at year 7. What is the weighted average life of each?Leverage, coverage and cash flowCoreScotiabankNew York · 2026

    Try it first

    Before calculating: roughly what is term loan B's weighted average life?

    Show the worked solution

    Term loan A has a weighted average life of 3.0 years and term loan B about 6.8 years. Term loan A repays 200 in each of years 1 to 5, so its average is (1 + 2 + 3 + 4 + 5) x 200 / 1,000 = 3.0. Term loan B repays 10 in each of years 1 to 6 and 940 in year 7: (210 + 6,580) / 1,000 = 6.79. Amortisation, not maturity, sets how long the lender's money is out.

    What are the two loans, and why do they amortise so differently?

    That is the question a candidate reported being asked, and the arithmetic is the best way to answer it. A term loan A is built for banks: it amortises steadily, so the lender's exposure shrinks every year. A term loan B is built for institutional investors such as loan funds: it amortises only nominally and returns almost everything at maturity, which suits investors who want to stay invested. The difference in structure shows up as a difference in weighted average life, and that number drives pricing, investor appetite and how long the credit risk lasts.

    Where the principal comes back sets how long the money is outTerm loan A: 20% a yearWAL 3.0 years2002002002002001234567yearTerm loan B: 1% a year, then bulletWAL 6.79 years1010101010109401234567year
    Term loan A returns 200 of principal every year and has a weighted average life of 3.0 years, while term loan B returns only 10 a year before a 940 bullet in year 7, giving a weighted average life of 6.79 years.

    How is weighted average life calculated?

    Think of lending a friend Rs 1,000 who repays Rs 200 a year: on average your money is out about three years, even though the last rupee comes back in year five. Weighted average life weights each repayment date by the share of principal repaid on it, and ignores interest. For term loan B: six payments of 10 contribute 10 x (1 + 2 + 3 + 4 + 5 + 6) = 210 year-rupees, and the bullet contributes 940 x 7 = 6,580. Total 6,790 over 1,000 is 6.79 years.

    The relationship
    WAL=∑tt⋅Pt∑tPt:A=200(1+2+3+4+5)1,000=3.0B=10(1+⋯+6)+940×71,000=6.79WAL = \frac{\sum_t t \cdot P_t}{\sum_t P_t}: \quad A = \frac{200(1+2+3+4+5)}{1{,}000} = 3.0 \qquad B = \frac{10(1+\dots+6) + 940 \times 7}{1{,}000} = 6.79
    P_tprincipal repaid in year t
    tthe year of repayment
    What it says in wordsWeighted average life is the average repayment date, weighted by how much principal comes back on each date.

    Why does the desk care?

    A loan's pricing is spread over its weighted average life, not its stated maturity, so an upfront fee or discount is spread over 3 years on term loan A and nearly 7 on term loan B. In practice term loans B are often repaid early from refinancings or cash sweeps: if this one were refinanced in full at year 4, its weighted average life would fall to 3.94 years. Weighted average life is also not duration; it ignores interest and discounting, so it always sits above the loan's duration.

    Where candidates lose it

    The common error is quoting maturity: 5 years and 7 years. The question is about how long the money is out on average, and the answer for term loan A is two years shorter than its maturity.

    The second is guessing term loan B's life at half its term because it amortises. It amortises so little that the bullet dominates. Say the 94% bullet share first and the answer follows.

    What the interviewer asks next

    • If term loan A amortised 5%, 10%, 15%, 20% and 50% over the five years, what is its weighted average life?
    • Why do institutional loan investors prefer the term loan B structure?
    • A 2 point upfront fee is spread over the weighted average life. What does it add to the yield on each loan?

    Asked at Scotiabank, Debt Capital Markets, New York, 2026 (Wall Street Oasis): Tell me about the two different types of loans in the broadly syndicated loan market.

  7. 097You are selling a loan and will receive five bids one at a time, in random order. You must accept or reject each bid on the spot, and a rejected bid never comes back. What rule maximises your chance of accepting the single best bid, and what is that chance?Probability and expected valueHardSyndicate desksCorporate banking

    Try it first

    Which rule gives the best chance of ending with the top bid?

    Show the worked solution

    Reject the first two bids, then accept the first bid that beats both of them; you get the best bid 43.3% of the time. Taking the first or last bid wins only 20%. The two rejected bids set a benchmark, and the rule succeeds whenever the best bid comes later and the best of the bids before it sits among the first two. Checking all 120 orderings gives 52 wins, which is 43.3%.

    Why reject bids you know nothing wrong with?

    House hunting in a city you do not know, you would look at a couple of flats before signing anything, simply to learn what good looks like. Sign too early and you never had a benchmark; look too long and the best one may already be gone. The rejected bids are the price of information: they set the bar that later bids must clear, and the only question is how many to spend. With five bids, spending two is the best trade.

    Look at two bids, then leap at the first one that beats themBid 1look, rejectBid 2look, rejectBid 3take if a recordBid 4take if a recordBid 5take if a recordset the barfirst bid above the bar wins the loanChance of picking the best bid, by bids rejected first20.0%reject 041.7%reject 143.3%reject 235.0%reject 320.0%reject 4
    Rejecting the first two bids and then taking the first bid that beats them picks the best of five bids 43.3% of the time, against 20% for taking the first or the last bid and 41.7% or 35.0% for rejecting one or three.

    How do you get 43.3% without listing all 120 orders?

    Ask where the best bid sits. If it is in the first two, you have already rejected it and lose. If it sits at position j, from 3 to 5, you take it only if no earlier bid after the first two already beat the bar, which happens when the best of the first j minus 1 bids lies in the first two. That chance is 2 out of (j minus 1), so the rule wins with probability one fifth of (2/2 + 2/3 + 2/4), which is 43.3%. Say the structure; the arithmetic takes ten seconds.

    The relationship
    P(best)=15(22+23+24)=15×2.167=0.433P(\text{best}) = \frac{1}{5}\left(\frac{2}{2} + \frac{2}{3} + \frac{2}{4}\right) = \frac{1}{5} \times 2.167 = 0.433
    1/5the chance the best bid is in any given position
    2/(j-1)the chance that, with the best bid at position j, the best earlier bid is among the two rejected
    What it says in wordsAdd up, over each place the best bid could arrive, the chance the rule is still waiting when it gets there.

    What is the limitation for a real loan sale?

    Two things. The rule maximises the chance of the very best bid, not the expected price; a seller who cares about the average price would behave differently. And real loan sales rarely force on-the-spot decisions: a desk runs a process that collects bids together, precisely to avoid this problem. With many bids, the rule becomes the well-known look at about 37% and then leap, and the success rate falls towards about 37% too.

    Where candidates lose it

    The common answer is to take the first good-looking bid. It wins only 20% of the time, because a good-looking bid with no benchmark is just a random bid.

    The second loss is knowing the 37% rule and applying it blindly: 37% of five is 1.85 bids, and the candidate who rounds without checking may reject one instead of two. Compute the small case directly; it takes a few lines.

    What the interviewer asks next

    • With ten bids, how many would you reject first?
    • If you are paid the bid you accept rather than rewarded only for the best, does the rule change?
    • Rejected bidders may come back with 50% probability. How does that change your cut-off?
  8. 098You have 1,000 bottles of wine and exactly one is poisoned. The poison shows its effect after 24 hours, and you have one day to find the bottle. What is the fewest testers you need, and how do you assign the bottles to them?Logic and brainteasersHardSyndicate desks

    Try it first

    How many testers do you need?

    Show the worked solution

    10 testers. Number the bottles 1 to 1,000 and write each number in binary with ten digits. Tester n drinks from every bottle whose n-th binary digit is 1. After 24 hours, the testers who fall ill spell out the poisoned bottle's number in binary. Ten testers give 1,024 possible patterns, enough for 1,000 bottles; nine give only 512.

    Why is each tester worth a binary digit?

    Think of the game twenty questions: every yes-or-no answer halves the possibilities, so twenty answers can pick out one thing in about a million. A tester is one yes-or-no answer: ill or fine. With one round of results, n testers can produce 2 to the power n different patterns, and you need at least as many patterns as bottles. 2^9 = 512 falls short of 1,000; 2^10 = 1,024 covers it. So ten is both enough and the minimum.

    Each tester is one binary digit; the ones who fall ill spell the bottleT10512T9256T8128T764T632T516T48T34T22T11Bottle 10000000001Bottle 20000000010Bottle 30000000011Bottle 5000111110100Bottle 6131001100101Bottle 10001111101000shaded = drinksTesters 10, 7, 6, 3, 1 fall ill: 512 + 64 + 32 + 4 + 1 = 61310 testers give 2^10 = 1,024 patterns, enough for 1,000 bottles; 9 give only 512
    Each bottle number written in ten binary digits tells each tester whether to drink from it, and if bottle 613 is poisoned, testers 10, 7, 6, 3, 1 fall ill, whose place values add back to 613.

    How do you read the answer back?

    Give tester n the place value 2 to the power (n minus 1): tester 1 is worth 1, tester 2 worth 2, up to tester 10 worth 512. Add the place values of the testers who fall ill and you have the bottle number. If bottle 613 is poisoned, its binary form is 1001100101, so testers 10, 7, 6, 3, 1 fall ill, and 512 + 64 + 32 + 4 + 1 = 613. Numbering 1 to 1,000 fits in ten digits because 1,000 is below 1,024, and the pattern where nobody falls ill is left spare.

    The relationship
    29=512<1,000≤1,024=2102^{9} = 512 < 1{,}000 \le 1{,}024 = 2^{10}
    2^nthe number of distinct ill-or-fine patterns n testers can produce
    What it says in wordsTen testers are the fewest whose outcomes can label every bottle.

    The desk version of the lesson: design the test so that every outcome carries information. A tester who drinks from one bottle learns about one bottle; a tester who drinks from half of them splits the problem in two.

    Where candidates lose it

    The common answers are 1,000 testers, one bottle each, or some splitting scheme that needs several rounds. The 24 hour delay allows only one round, so the testers must be designed to answer in parallel.

    The second loss is giving 10 without the proof that 9 is not enough. Say 2^9 is 512 in the same breath; it turns a remembered trick into an argument.

    What the interviewer asks next

    • You have two days instead of one, and a tester who falls ill on day one is out. How many testers do you need?
    • Exactly two bottles are poisoned. Does the binary scheme still work?
    • How many testers would a million bottles need?
  9. 099At initial price thoughts of 150 basis points over the benchmark, a bond's order book is 4 times covered. Each 5 basis points of tightening loses 10% of the original demand. How far can you tighten and still keep the book at least 2 times covered?Issuance and refinancing arithmeticCoreSyndicate desks

    Try it first

    Where does the book hit 2 times cover?

    Show the worked solution

    You can tighten 25 basis points, to 125, and still be 2 times covered. Each 5 basis point step loses 10% of the original demand, which is 0.4x of cover. Going from 4.0x to 2.0x gives up 2.0x, or five steps, so 5 x 5 = 25 basis points. On a Rs 500 crore deal the book falls from Rs 2,000 crore to Rs 1,000 crore, and the issuer saves Rs 1.25 crore a year.

    What is actually being traded off?

    A shopkeeper with a queue of forty customers for twenty items can raise the price, and some customers will walk away; the question is how high to go before the queue gets too short to be comfortable. Book-building trades cover for price: each step tighter saves the issuer money and costs orders, and cover is the buffer against a weak aftermarket. Here every 5 basis points costs 0.4x, because the loss is 10% of the original book each time.

    Each 5 bp of tightening costs 0.4x of cover1501451401351301251201150.0x1.0x2.0x3.0x4.0xSpread over the benchmark, bp (tighter to the right)2.0x floor4.0x3.6x3.2x2.8x2.4x2.0xstop at 125Rs 500 crore deal, book Rs 2,000 croreeach step: -Rs 200 crore of orders25 bp tighter saves Rs 1.25 crore a year
    Starting at 4.0x cover at 150 basis points, each 5 basis points of tightening removes 0.4x, so the book reaches the 2.0x floor at 125 and any further tightening leaves it under-covered.
    The relationship
    cover(s)=4.0−0.4×150−s5≥2.0  ⇒  150−s≤25\text{cover}(s) = 4.0 - 0.4 \times \frac{150 - s}{5} \ge 2.0 \;\Rightarrow\; 150 - s \le 25
    sthe final spread in basis points
    0.4the cover lost per 5 bp step, 10% of the original 4.0x
    2.0the minimum cover you want to keep
    What it says in wordsCover falls by the same amount every step, so divide the cover you can spare by the cover each step costs.

    Would you really go all the way to 125?

    Not automatically. The linear rule is a model of the book; real books drop in lumps, because price-sensitive orders carry limits and the orders that leave first are often the highest quality long-term investors. Landing exactly on the 2.0x floor leaves no margin if a few orders drop after the final terms. A syndicate desk might stop one step short, at 130 with 2.4x cover, giving up Rs 0.25 crore a year of saving on Rs 500 crore for a safer book and a better chance of a positive break. The arithmetic sets the limit; the judgement picks the point inside it.

    Where candidates lose it

    The common slip is treating 10% as 10% of the remaining book, which makes each step smaller and the answer wider than 25 basis points. The question says original demand, so the steps are equal.

    The second loss is stopping at the number. The interviewer wants to hear what cover is for, why the real book is lumpier than the model, and why a desk might leave some tightening unused.

    What the interviewer asks next

    • If each step lost 10% of the remaining book instead, how far could you tighten?
    • What else in the book, besides its size, would you look at before tightening?
    • Why might an issuer prefer a larger deal at 130 to the planned size at 125?
  10. 1004% of issuers in a sector default within a year. An early warning model flags 75% of the issuers that will default and 10% of those that will not. An issuer is flagged. What is the probability that it defaults?Probability and expected valueCoreBelvedere TradingChicago · 2022

    Try it first

    Pick the closest before you calculate.

    Show the worked solution

    About 23.8%. Picture 1,000 issuers: 40 will default and 960 will not. The model flags 75% of the 40, which is 30, and 10% of the 960, which is 96. Of the 126 flagged issuers, 30 default, so the chance is 30 / 126 = 23.8%. The flag multiplies the risk about six times, but most flags are still false alarms because defaults are rare.

    Why is the answer not 75%?

    A smoke alarm that goes off for every real fire and also for one in ten batches of toast will ring mostly for toast, because toast is far more common than fire. The early warning model is the same. The chance of default given a flag depends on how common defaults are to begin with, and when the base rate is low, false flags from the large healthy group swamp the true flags from the small defaulting group. 75% answers a different question: how often a defaulter gets flagged.

    Out of 1,000 issuers, most flags land on companies that are fine1,000 issuers40 will default4%960 will not96%30 flagged75%10 missed25%96 flagged10%864 cleared90%An issuer is flagged. It is one of 30 + 96 = 126 flagged issuers.Only 30 of them default:30 / 126 = 23.8%most flags are false alarms
    Of 1,000 issuers, the 40 that will default produce 30 flags and the 960 that will not produce 96 false flags, so only 30 of the 126 flagged issuers default, a probability of 23.8%.
    The relationship
    P(D∣F)=P(F∣D)P(D)P(F∣D)P(D)+P(F∣Dˉ)P(Dˉ)=0.75×0.040.75×0.04+0.10×0.96=30126=23.8%P(D \mid F) = \frac{P(F \mid D)P(D)}{P(F \mid D)P(D) + P(F \mid \bar D)P(\bar D)} = \frac{0.75 \times 0.04}{0.75 \times 0.04 + 0.10 \times 0.96} = \frac{30}{126} = 23.8\%
    P(D)the base rate of default, 4%
    P(F | D)the chance a defaulter is flagged, 75%
    P(F | not D)the chance a healthy issuer is flagged, 10%
    What it says in wordsOf all the flags, the share that come from real defaulters is the chance a flag means default.

    How should a credit desk use a 23.8% flag?

    As a reason to look harder, not a verdict. A flag lifts the default probability from 4% to 23.8%, roughly six times, which is worth a review but not a sale on its own. A second, independent signal compounds the evidence: if a separate test with the same accuracy also flagged the issuer, the probability would rise to about 70%. The honest limitation is that two warning signals about the same company are rarely independent, so the real lift from a second flag is usually smaller.

    Where candidates lose it

    The common error is answering 75%, swapping the probability of a flag given default for the probability of default given a flag. It is the most frequent mistake in Bayes questions, and interviewers set it up deliberately.

    The second loss is doing the formula silently and producing 23.8% with no picture. Say the 1,000 issuers out loud, 30 true flags and 96 false ones, and the interviewer can follow every step.

    What the interviewer asks next

    • What false alarm rate would the model need for a flag to mean a 50% chance of default?
    • The sector's default rate doubles to 8%. What does a flag mean now?
    • Two independent models both flag the issuer. What is the probability of default?

    Asked at Belvedere Trading, Capital Markets, Chicago, 2022 (Wall Street Oasis): The technical portion of the interview consisted of probability questions including one questions relating to Bayes' theorem

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