each digit can be 0-9 ( 10 numbers)
there are 9 numbers in a social security number
multiply 10 nine times:
10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 x 10 = 1,000,000,000 ( 1 billion) possibilities.
the restriction is it can't be all zeros so subtract 1 from the total
1,000,000,000 - 1 = 999,999,999 combinations
