Please show work A mailman has special delivery mail for 9 c

Please show work.

A mailman has special delivery mail for 9 customers. In how many ways can he arrange his schedule to deliver to all 9? In how many ways can he schedule deliveries if he can deliver to only 7 of the 9?

Solution

a)

No. of customers are 9 and we suppose to fill 9 blanks. First blank can be filled in with any of 9 choice, second blank can be fill with remaining of 8 choice and so on..Each of the blank is filled in possible arrangment is 9 * 8 * 7 * 6 * 4 *3 *2 *1 = 362880

b)

It can be delivere to only 7 of the 9 will be 9 C 7 = 9! / (9-7)! * 7! = 36 ways