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
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.
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.