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Can you help me with this problem?

There are $7$ married couples. What will be the number of mixed double pairs in tennis such that no one plays with his/her spouse?

Can some one help me with this? The answer is $840$.

Thanks!

EDIT : The question asks for the number of teams possible in which no wife is teamed up with her husband.

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    I don't think this problem is clear enough. I thought a "mixed doubles pair" was just a male and female pairing, but there are only $42$ such pairings. Do you mean that no wife plays with her husband as a partner, or that no wife plays on the opposing side against her husband, or that no wife plays on either side, either with or against her husband? Given the answer is 840, I think the question means the latter.2011-10-25
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    (Whoops, editing time ran out.) Insert before my questions the sentence: "Presumably you want the number of pairs of pairs that can participate in match. Do you want ..."2011-10-25
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    I don't think not playing with your spouse is a very reasonable constraint for a tennis tournament. But I can conceive other social events where it makes sense...2012-03-22

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