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Cards between the first two and the first ace

Question

A standard 5252-card deck is shuffled and dealt face up one card at a time. Suppose the first 22 comes out before the first ace. Under this condition, what is the expected number of cards dealt strictly between the first 22 and the first ace?

Short answer

37945\tfrac{379}{45} (about 8.42). The 8 aces and 2s split the other 44 cards into 9 gaps of average size 449\tfrac{44}9. Given a 2 leads, the remaining three 2s fall before the first ace on average 35\tfrac35 times, so 85\tfrac85 gaps are spanned. The answer is 85⋅449+35=37945\tfrac85\cdot\tfrac{44}9+\tfrac35=\tfrac{379}{45}.

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

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