4 friends are going to sit together in a roller-coaster car that has 4 seats, in how many different arrangements can they sit? (I've done this equation before, it's very simple)
4 people can go in the first seat
there are 3 left to pick for the nest
then, 2 left to pick for the last
then 1 for the final one
so there are 4 x 3 x 2 x 1 choices