Investment Banking interview preparation
Every question below is either traced to a named firm from a public candidate report, or tagged at desk level when we could not trace it. Answers are written the way you would actually say them out loud — answer first, then the mechanism, then the limitation.
100 questions, mapped to the firms that asked them
- Questions
- 100
- Traced to a firm
- 100
- Firms
- 46
- Updated
- September 2026
090You have a rope 30cm long and another 45cm long. Each centimetre takes one minute to burn. How do you measure exactly 25 minutes?Moelis & CompanyInvestment Banking · New York · 2025
Say this
The only tool you have is that lighting a rope at both ends halves its time. With these two lengths the times you can construct are 15, 22.5, 26.25, 30 and 37.5 minutes — 25 is not one of them. I would show you that working and tell you the puzzle as stated has no exact solution.
Then walk it
- The single insight: a rope lit at both ends burns out in half its length in minutes, regardless of where it burns unevenly.
- Light the 30cm at both ends and it is gone at 15 minutes. Light the 45cm at both ends and it is gone at 22.5.
- Best combination for something near 25: light the 45cm at both ends and the 30cm at one end at time zero. At 22.5 minutes the 45 is gone and the 30 has 7.5cm left. Light its second end and it burns out 3.75 minutes later, at 26.25.
- The other combination goes the wrong way: light the 30cm at both ends and the 45cm at one end. At 15 minutes the 45 has 30cm left; lighting its second end gives 15 more minutes, so you land on 30.
- So the reachable set is 15, 22.5, 26.25, 30, 37.5 and the un-halved 45. There is no route to exactly 25.
- In the room, say the halving principle immediately, construct the timings out loud, then say plainly that 25 is not reachable and 26.25 is the closest. Interviewers garble the lengths on this puzzle constantly, and catching that scores better than forcing a wrong answer.
Where candidates lose it
Forcing an answer because you assume the question must have one. Narrate the halving principle, build the reachable times, and say if the target is not among them. Confidently asserting a solution that your own arithmetic contradicts is the actual failure here.
Expect next
- What is the general set of times you can construct?
- How much water can you measure with a 3-litre and a 4-litre bottle?
- What angle do the clock hands make at 3:15?
Reported by candidates at Moelis & Company (Investment Banking, New York, 2025). Source: Wall Street Oasis.
091What angle is formed by the clock hands at 3:15?Centerview PartnersInvestment Banking · Menlo Park · 2025
Say this
7.5 degrees. The minute hand is exactly on the 3, but the hour hand has moved a quarter of the way from 3 to 4. Each hour is 30 degrees, so a quarter of that is 7.5.
Then walk it
- The clock face is 360 degrees over 12 hours, so each hour mark is 30 degrees apart.
- At 3:15 the minute hand sits exactly at the 3, which is 90 degrees from twelve.
- The hour hand does not stay on the 3. In 15 minutes it travels a quarter of the way to the 4, which is a quarter of 30 degrees, so 7.5 degrees past the 3.
- The gap between them is therefore 7.5 degrees.
- The general formula worth having: the angle equals the absolute value of 30 times the hour minus 5.5 times the minutes. Here that is 90 minus 82.5, which is 7.5.
Where candidates lose it
Answering zero, because you assumed both hands are on the 3. The hour hand moves continuously, and that is the entire point of the question. Say the formula afterwards to show it was not luck.
Expect next
- When is the next time the hands overlap exactly?
- How many times a day do the hands overlap?
- How many 0s are in 1000 factorial?
Reported by candidates at Centerview Partners (Investment Banking, Menlo Park, 2025). Source: Wall Street Oasis.
092I paint the surface of a 10x10x10 cube green. How many of the small cubes have green on them, and how many do not?TD SecuritiesGeneralist · New York · 2024
Say this
488 have green on them and 512 do not. Solve the easy half first: the unpainted cubes form an inner 8x8x8 block, which is 512. Subtract that from 1,000 to get 488 painted.
Then walk it
- Total small cubes: 10 cubed is 1,000.
- Any cube not touching the surface is one layer in from every face. That leaves an 8 by 8 by 8 core, because you remove one layer from each of the two opposite sides in all three dimensions.
- 8 cubed is 512 unpainted.
- So painted is 1,000 minus 512, which is 488. Computing the painted faces directly is possible but far more error-prone.
- If they push further: 8 corner cubes have three painted faces, 96 edge cubes have two, and 384 face cubes have one. That sums to 488, which confirms the answer.
Where candidates lose it
Trying to count the painted cubes face by face and double-counting the edges and corners. Always solve for the interior and subtract. Saying 'I will do the easy side first' out loud is itself the signal they want.
Expect next
- How many have exactly two painted faces?
- Generalise it for an n-cube.
- Now do it for a 10x10x10 where I only paint the top and bottom.
Reported by candidates at TD Securities (Generalist, New York, 2024). Source: Wall Street Oasis.
093Lilypads double every day and cover the pond on day 30. On what day was the pond half covered?Harris WilliamsGeneralist · Richmond · 2025
Say this
Day 29. If the area doubles every day, then the day before it was full it must have been exactly half. Work backwards, not forwards.
Then walk it
- Doubling means the step from half to full takes exactly one day.
- Full on day 30 therefore means half on day 29.
- The instinct to halve the number of days and say 15 is the trap. On day 15 the pond is about one thirty-two-thousandth covered, since it doubles fifteen more times.
- The general principle: in exponential growth, almost all of the accumulation happens at the very end. That is why the answer feels wrong.
- It is worth naming the finance version, because that is why the question gets asked: compounding, and the danger of extrapolating an exponential trend from a small base.
Where candidates lose it
Saying day 15. It is the reflex answer and it is wrong by a factor of thirty thousand. Answer in one second, then add the exponential-growth observation so it reads as understanding rather than recall.
Expect next
- What does that tell you about compounding?
- If I make 8 times my money in 6 years, what is my IRR?
- How would you spot a business at the steep part of that curve?
Reported by candidates at Harris Williams (Generalist, Richmond, 2025). Source: Wall Street Oasis.
094How many aeroplanes are in the air at once?Société GénéraleMergers and Acquisitions · New York · 2025HSBCSales and Trading · Hong Kong · 2026
Say this
Around ten to fifteen thousand. Build it from the fleet: roughly 25,000 commercial aircraft worldwide, each flying maybe 10 hours a day, so about 40 percent are airborne at any moment, which gives 10,000.
Then walk it
- Start from a number you can defend: the global commercial fleet is on the order of 25,000 aircraft. If you do not know it, build it from airlines: a few hundred carriers averaging perhaps 80 aircraft.
- Utilisation: a commercial jet flies roughly 8 to 12 hours a day. Call it 10, which is about 40 percent of a 24-hour day.
- 25,000 times 0.4 gives 10,000 airborne on average.
- Then adjust for the peak-trough pattern. Air traffic is concentrated in daylight hours across the Northern Hemisphere, so the daytime peak might be 50 percent above average and the small hours well below. Peak perhaps 15,000.
- Then say what you excluded and why: private aviation, cargo and military would add to it, probably a few thousand more, and I have not counted them.
- Close with the sanity check: published flight-tracking figures sit in the same range, low to mid five figures, so the estimate holds.
Where candidates lose it
Guessing a number with no derivation, or getting lost in decimals. Market sizing is judged entirely on structure. State each assumption, keep the arithmetic round, and say which populations you left out.
Expect next
- How many taxis are in Hong Kong Central?
- How many hotel beds are there in New York City?
- How many books were sold in the US last year?
Reported by candidates at Société Générale (Mergers and Acquisitions, New York, 2025); HSBC (Sales and Trading, Hong Kong, 2026). Source: Wall Street Oasis.
095What is 301 times 447?General AtlanticGeneralist · New York · 2026
Say this
134,547. Break 301 into 300 plus 1. Three hundred times 447 is 134,100, plus one more 447 gives 134,547.
Then walk it
- Decompose to the nearest round number. 301 is 300 plus 1, and 300 is easy because it is 3 times 100.
- 447 times 3 is 1,341. Times 100 is 134,100.
- Add the remaining 447: 134,547.
- Announce the decomposition before you compute, so the interviewer can follow and so a slip in arithmetic does not look like a failure of method.
- The same trick handles most of these: round one number, multiply, then correct. For 19 times 63, do 20 times 63 minus 63.
Where candidates lose it
Attempting long multiplication in your head and losing a digit. These are speed-and-method tests. Decompose out loud, keep one running total, and offer the sanity check that the answer should be a bit over 134,000.
Expect next
- What is 19 times 63?
- What is 1/7 as a decimal?
- What is the square root of 6,241?
Reported by candidates at General Atlantic (Generalist, New York, 2026). Source: Wall Street Oasis.
Firm tags come from public, anonymous candidate reports on Wall Street Oasis: strong signal, not sworn testimony. Firms are named as the places a question was reported, not as partners of Fin Maverick. Answers are written for this page to show how to think out loud; they are not scripts to recite.