Hello!
By finding the probability, we will see that Carter will have selected one out of the amount of possible locker combinations.
First, we have 1/13 twice for the 1st and 3rd numbers, and for the second 1/9 counting zero. If we multiply 1/13, 1/13, and 1/9, we get 1/1521.
This means that there were 1521 total locker combinations Carter could choose from.
Hope this helps!