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Best stopping card for an ace-placement prize

Question

Alice chooses a number kk from 11 to 5252, then turns over the top kk cards of a well-shuffled standard deck. She wins if the kkth card she turns is an ace and exactly one ace remains among the 52−k52-k unturned cards. Which kk maximises her chance of winning?

Short answer

k=35k=35. Winning needs two aces among the first k−1k-1 cards, an ace at position kk, and one ace in the rest, so the probability is proportional to (k−1)(k−2)(52−k)(k-1)(k-2)(52-k). The continuous maximiser is about 35.235.2; comparing integers, k=35k=35 gives 1907419074 versus 1904019040 at k=36k=36.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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