Telephone Numbers. In the United States, telephone numbers consist of a three-digit area code followed by a seven-digit local number. Suppose neither the first digit of an area code nor the first...

Telephone Numbers.
In the United States, telephone numbers consist of a three-digit area code followed by a seven-digit local number. Suppose neither the first digit of an area code nor the first digit of a local number can be a zero but that all other choices are acceptable.

a. How many different area codes are possible?
b. For a given area code, how many local telephone numbers are possible?
c. How many telephone numbers are possible?