Question

A certain​ country's postal service currently uses 55​-digit zip codes in most areas. how many zip codes are possible if there are no restrictions on the digits​ used? how many would be possible if the first number could not be 00​?

Answer 1

There would be 100,000 if there are no restrictions on the digits and 90,000 if they cannot use 0 as the first digit.

If there are no restrictions, there are 10 possibilities for each digit:
10(10)(10)(10)(10) = 100,000

If the first digit cannot be 0, there are 9 possibilities for it and 10 possibilities for each of the other 4:
9(10)(10)(10)(10) = 90,000