Dealing until two kings or a king and an ace
Question
A well-shuffled -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
. 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 on average (independent of labels), while given an ace comes first the first king averages position . Each case has probability , so the mean is , confirmed by exact recursion.
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Open the worked solutionLessons behind this question
- Conditional Probability, Total Probability, and Bayes' Rule
- Worked Examples — Probability Trees, Monty Hall, and Bayesian Updating
- Expectation, Mean, and Variance
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