Question

Social Security: Find the number of possible 9-digit social security numbers if the digits may be repeated.

Answer 1

10 possible digits, in 9 different places, so you should have \[10^9\]different possibilities.

Answer 2

but for the explanation part ssn's look like: xxx-xx-xxxx
that first x can be any digit from 0,1,2,3,4,5,6,7,8,9 -- so 10 choices
the next x can also be 0,1,2,3,4,5,6,7,8,9 -- between the two you can have 100 different combinations 00,01,02...97,98,99 (it is 100 because the count starts at 00) and so on for each additional x.

So for shorthand use George's answer