A man has 9 friends: 4 boys and 5 girls. In how many ways can he invite them, if there have to be exactly 3 girls in the invitees? (CAT 1996)
Option (b) 3 girls can be selected out of 5 girls in 5C3 ways. Since the number of boys to be invited is not to be given, he can invite or not invite them in 24 ways.
Hence required numbers of ways =5C3 × 24= 10 × 16 = 160.