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Dealing until two kings or a king and an ace

Question

A well-shuffled 5252-card deck is turned over one card at a time. Dealing stops the moment either a second king has appeared, or at least one king and at least one ace have both appeared, whichever comes first. What is the expected number of cards turned over?

Short answer

121990≈13.54\tfrac{1219}{90}\approx13.54. Dealing always ends at the second card from the set {kings, aces} unless the first such card is an ace, in which case it ends at the first king. By the equal-gaps argument the second of the 8 special cards sits at position 1069\tfrac{106}{9} on average (independent of labels), while given an ace comes first the first king averages position 68945\tfrac{689}{45}. Each case has probability 12\tfrac12, so the mean is 12(1069+68945)=121990\tfrac12\big(\tfrac{106}{9}+\tfrac{689}{45}\big)=\tfrac{1219}{90}, confirmed by exact recursion.

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

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