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Second pair from a 40-card deck missing a pair

Question

A deck has 4040 cards: four copies each of the numbers 11 through 1010. One pair of cards with the same number is taken out of the deck. Two cards are then drawn at random from the remaining cards. What is the probability that these two cards also have the same number?

Short answer

55/70355/703. After the pair is removed, 3838 cards remain: nine numbers still have four copies and one has two. Matching pairs number 9(42)+(22)=559\binom42+\binom22=55 out of (382)=703\binom{38}{2}=703.

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

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