160.Expected Pairs in Five Cards
★★★expected valueSIG
A deck holds ten cards: two tens, two jacks, two queens, two kings and two aces. Five cards are dealt without replacement. What is the expected number of pairs in the hand, where a pair means both cards of a rank are present?
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Put an indicator on each rank and ask how often both of its cards land in the hand. Do not try to enumerate hands.
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