The set S has 36 different subsets each of which contains exactly two elements. How many subsets of S could contain exactly four elements each?	Answers
A) 126 B) 120 C) 110 D) 100 E) 118
nC2 = 36   =&gt; n*(n-1)/2 = 36 by middle term factor n can not be negative   =&gt; n = 9   nC3 = 9C4 = 126   So, 126nswer is 126.
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The set S has 36 different subsets each of which contains exactly two elements. How many subsets of S could contain exactly four elements each?