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Four of a kind given at least three of a kind

Question

A five-card hand is dealt from a standard 5252-card deck. You are informed that at least three of the cards share a rank. What is the conditional probability that the hand holds all four cards of some rank?

Short answer

1/951/95. Four-of-a-kind hands: 13⋅48=62413\cdot48=624. Hands with exactly three of a rank: 13(43)(482)=5865613\binom43\binom{48}{2}=58656. The answer is 624624+58656=195\tfrac{624}{624+58656}=\tfrac{1}{95}.

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

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