Classic quant interview problems, solved deeply.
The specific problems that show up again and again in quant interviews. Each page has the full canonical solution, an AI-generated alternative framing, common variations, and the firms that ask it.
Reject the first 1/e of candidates, then take the best so far. The classic optimal-stopping problem and its famous 1/e answer.
Always switch. The counter-intuitive 2/3 answer and why the host's knowledge matters.
At least one is a girl. What's the probability both are? Not 1/2 — the classic conditioning trap.
Expected number of draws to collect all n types. The answer is n · H_n ≈ n ln n.
Starting with £k, what's your chance of reaching £N before going broke? The canonical absorbing-chain problem.
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