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

The number of subsets of A containing exactly three elements is  nC3 whereas the number of three subsets of 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