Akey code must contain 6 numbers. there are 10 numbers available. how many different key codes can be created?

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 .