**Q. **A man has nine friends - four boys and five girls. In how many ways can he invite them, if there have to be exactly three girls in the invitees? **(CAT 1996)**(Given possible answer choices - 320, 160, 80, 200)

**Ans.**

This is a pure selection problem, the arrangement is inconsequential. If 3 girls are selected from 5 girls, then the arrangement of the three girls is insignificant and only the selection is important. Thus **Combination** is involved. Three girls always have to be...