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There are n pairs of shoes in a closet. ...

There are n pairs of shoes in a closet. If `r(ltn)` are selected at random then the number of ways that among the selected shoes
there are exactly 2 pairs is `""^(n)C_(2).""^(n-2)C_(r-4).2^(r-4)`

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`=""^(n)C_(2).""^(n-2)C_(r-4).2^(r-4)`
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