a)

total of 40 players are grouped into two..

players can be sorted into ordered groups in ways.

now find the number of ways players can be sorted intounorderd pairs......(I)

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also, a division will give you no offensive-defensive pairs if the offensive (and therefore the defensive) players are paired among themselves. So there are divisions for offensive as well as defensive players...(II)

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Using (I) and (II), you can now find the probability that there are no offensive-defensive pairs!