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Probability of exactly one pair in five-card poker

Question

Five cards are dealt from a well-shuffled standard 5252-card deck. Find the probability that the hand is exactly one pair: two cards share a rank and the remaining three cards have three other ranks, all different from each other.

Short answer

352833≈0.423\tfrac{352}{833}\approx0.423. Choose the paired rank and its suits, 13(42)13\binom42, then three other distinct ranks with any suits, (123)43\binom{12}{3}4^3. Dividing 13⋅6⋅220⋅64=1,098,24013\cdot6\cdot220\cdot64=1{,}098{,}240 by (525)=2,598,960\binom{52}{5}=2{,}598{,}960 gives 352833\tfrac{352}{833}.

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

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