First spot: 9 choices (0-9)
Second spot: 9 choices (0-9)
Third spot: 9 choices (0-9)
.
.
.
Ninth spot: 9 choices (0-9)
There are 9 choices for each digit, meaning we need to compute 9^9 to figure out how many possible social security numbers there are.
Explination:
9^9 = 387420489 possible social security numbers