Quant Interview Brain Teasers

Six classic brain teasers, each labeled with the trading firm it's most commonly attributed to in public interview-prep discussions — Jane Street, Two Sigma, Citadel, Optiver, D.E. Shaw, and SIG. Try each one before revealing the answer — free, no signup.

Jane Street

The 100 Passengers Problem

100 passengers board a plane with 100 assigned seats, one each. The first passenger lost their boarding pass and sits in a random seat. Every passenger after that sits in their own seat if it's free — otherwise, they pick a random open seat. What's the probability the 100th passenger ends up in their own assigned seat?

Two Sigma

The Birthday Match

How many people need to be in a room before there's a better-than-even chance that two of them share a birthday?

Citadel

The Poisoned Bottle

You have 1,000 bottles of wine for a big event tomorrow. Exactly one is poisoned. Any amount of the poison causes symptoms in a test rat after exactly 24 hours, and you have exactly 24 hours before the event starts. What's the minimum number of rats you need to guarantee finding the poisoned bottle in time?

Optiver

Roll Until You've Seen Them All

You roll a fair six-sided die repeatedly until every one of the six faces has come up at least once. What's the expected number of rolls?

D.E. Shaw

The Two Guards

Two guards stand in front of two doors. One door leads to a fortune, the other to nothing. One guard always tells the truth and the other always lies — but you don't know which is which. You may ask exactly one question, to exactly one guard, before choosing a door. What do you ask?

SIG (Susquehanna)

The Betting Game

I offer you this bet on a fair coin flip: heads, I pay you $3; tails, you pay me $1. Do you take it? And does your answer change if you could play it 100 times in a row instead of once?

Want more?

These six are just a sample. The TradeMind app's Brain Teasers module has a much larger rotating set — logic puzzles, probability traps, and game-theory questions — plus hint tiers, a difficulty curve, and progress tracking across attempts.

FAQ

Are these the exact questions these firms actually ask?

Almost certainly not word-for-word. Each puzzle below is a classic that shows up again and again in public interview-prep discussions (Glassdoor, Wall Street Oasis, prep books) tagged to the firm named in its title — but the same underlying puzzle frequently gets reported across many different firms and even outside finance entirely. Treat the firm label as "this is where it's most often attributed," not a guarantee of current or exclusive use.

Why do quant firms ask brain teasers at all?

Less to check if you already know the trick, more to watch HOW you think out loud: do you reach for symmetry and expected value, do you sanity-check an answer against intuition, do you stay calm when a first instinct turns out wrong. The path to the answer is usually worth more than the answer itself.

What's the best way to practice these?

Cover the answer, set a 2–3 minute timer, and narrate your reasoning out loud as if an interviewer were listening — silently staring at a puzzle is a very different skill from talking through one under mild time pressure, and the second is what's actually being tested.

These puzzles are independent adaptations of classic brain teasers that circulate widely across public interview-prep sources, written in our own words for practice purposes. The firm name on each one reflects common public attribution, not a confirmed or exclusive source — the same puzzle frequently gets reported across many different companies, inside and outside quantitative finance. TradeMind is not affiliated with, endorsed by, or sponsored by Jane Street, Two Sigma, Citadel, Optiver, D.E. Shaw, Susquehanna (SIG), or any other firm named on this page. Firm names are used solely to describe how a given puzzle is commonly attributed in public sources.