There are 7 boys and 8 girls sitting in a row. In how many different ways can they sit if certain 3 girls always sit together?

Think 3 girls as one girl. Then we have 7 boys and 6. (8-3+1) girls so 13 people are here. We know 13 people sit in 13! different ways. But don't forget we get three girls as one and they can sit in 3! different ways. It means those all people can sit in 13! * 3! different way

Hi I cannot comment now but I will give you a hint. First create a three-girl seat, then the problem reduces to 7 boys and 6 girls sitting in a row. Then within that three-girl seat you have 3! ways of arranging the three girls.