6 choices for the first person; XXfor each of these, 5 choices for the next; XXXXfor each of these, 4 choices for the next; XXXXXXfor each of these, 3 choices for the next; XXXXXXXXfor each of these, 2 choices for the next; XXXXXXXXXXfor each of these, 1 choice for the last person.