A man has 9 friends: 4 boys and 5 girls. In how many ways can he invite them, if there has to be exactly 3 girls in the invitees?
Selecting 3 girls from 5 girls can be done in $$^5C_3$$ ways => 10 ways
Each of the boys may or may not be selected => 2 * 2 * 2 * 2 = 16 ways
=> 16 * 10 = 160 ways
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