3 Girls and 4 boys are to be seated in a row on 7 chairs in such a way that all the three girls always sit together. In how many different ways can it be done ?

Total number of persons = 3 girls + 4 boys = 7

The girls always sit together. Considering three girls as one person, we have 5 persons who can be arranged in 5! ways. But corresponding to each way of these arrangements, the girls can be arranged together in 3! ways.

Hence, required number of ways

= 5! × 3! = 120 × 6 = 720