UPSC NDA I 2015 Mathematics Solved Paper

Subsets of A containing exactly 2 elements can be formed in the following ways.
A={1,2,3,4,5,6,7,8,9,10}
 :
 :
 :
S9={9,10}→n=1
Therefore total number of subsets of A containing exactly two elements will be equal to
ntotal=

i=9

∑

i=1

ni
=

9(9+1)

2

=45