Each digit has 10 possible values (the numbers from 0 to 9), so in order how many different numbers can be formed, we need to multiply the number of possible values of each digit.
If we have 9 digits, and each digit has 10 possible values, we need to multiply the number 10 by itself 9 times, that is:
[tex]N=10^9[/tex]So the number of different social security numbers is 10^9 (1,000,000,000, one billion)
