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ilvpat
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Homework Statement
There are 10 sister/brother pairs, (S1,B1) ... , (S10,B10). We then divide the 20 people randomly into 10 (not necessarily female/male) pairs.
Let X be the number of female/male pairs. Find E(X)
Let Y be the number of pairs consisting of siblings (that is, sister got matched with her brother). Find E(Y)
Homework Equations
The Attempt at a Solution
I only know how to find out the total number of ways to select 10 pairs, that's (20,2)+(18,2)+(16,2)+...+(2,2). (x,y) means x choose y
Please help. Thanks!