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Odds against a four of a kind in five cards

Question

Five cards are dealt from a standard 5252-card deck. If pp is the probability that four of them share a rank, what is 1/p1/p?

Short answer

41654165. There are 1313 choices of rank for the four matching cards and 4848 choices for the fifth card, giving 624624 hands out of (525)=2,598,960\binom{52}5=2{,}598{,}960. The ratio is 14165\tfrac1{4165}.

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