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Question:

How many ways can 6 people sit on a bench if 3 of them have to sit side by side?

A 144
Explaination

Treat the 3 people who wish to sit side by side as a single unit. There are 4 units consisting of three people in the group. The 4 units can be arranged in 4! ways on the bench. The three people in the group can be arranged in 3! ways.