To elaborate on Brian's post, the way he got the 7*6*5 can be thought up a little easier if you do a small diagram
I made mine like each - is a seat, since people cant sit in 2 seats, each time the amount of the people who can sit in a seat goes down.
- - - - - - - -
8 7 6 5 4 3 2 1
amount of choices left for each seat
