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Combinations : Finding possible ways a social security number can be arranged.

1. A social security number has 9 digits. If numbers can be repeated and all numbers can be used, there would be 10^9 possible social security numbers. Find each of the following.
a. How many possible social security numbers are there if numbers can be repeated but 0 cannot be used for the first digit?

Solution Preview

0 can not be used for first digit, therefore, the number of ways by which the first digit can be formed by = 9 ways

Rest of the 8 digits can be formed by 10 ways.

Therefore, ...

Solution Summary

Combinations of digits in social security numbers are investigated.