If 20% of three subsets (i.e., subsets containing exactly three elements) of the set A=a1,a2,…,ancontain a1 then value of n is

The number of subsets of A containing exactlythree elements is  nC3 whereas the number of three subsetsof A that contain a1 is  n−1C2 we are given. n−1C2=20100 nC3⇒(n−1)(n−2)2=15n(n−1)(n−2)(6)⇒n=15.