Question

A key code must contain 6 numbers. There are 10 numbers available. How many different key codes can be created?

Answer 1

The first number in the code can be any one of the 10 available.
           For each of those . . .
The second number in the code can be any one of the remaining 9.
           For each of those . . .
The third number in the code can be any one of the remaining 8.
           For each of those . . .
.
.
The sixth number in the code can be any one of the remaining 5.

The total number of possible codes is  (10 x 9 x 8 x 7 x 6 x 5) =  151,200 .