Start with $50, bet $1 a flip on a fair coin, stop at $100 or $0. What are the odds of doubling up?

Mira starts with $50 and bets $1 a flip on a fair coin until she reaches $100 or $0. The story asks the probability she reaches $100 first. You type your answer and check it. Then you step through the answer and the breakdown.

Why quant interviews test this

Say 'fair game, so the expected value doesn't change', give i/N, and be ready for the unfair coin. The written solution is at /brain-teasers/gamblers-ruin/solution.

The The Gambler's Ruin guide covers how scoring works, the strategy that wins, a worked example and the mistakes most players make.

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