Since total number of subsets of the set S=220 Now, the sum of all number from 1 to 20=
20×21
2
=210 Then, find the sets which has sum 7 . (1) {7} (2) {1,6} (3) {2,5} (4) {3,4} (5) {1,2,4} Then, there is only 5 sets which has sum 203 Hence required probability =