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 .
