Probability of phone numbers

Question

I am having trouble figuring out how to solve this. Would you be able to help me?

Local phone numbers consist of seven numerals, the first three of which are common to many users.

A small town's phone numbers all start with 336, 364, or 375. How many phone numbers are available?

Answer 1

Since it has been established that there is only 3 number prefixes we just focus on the the 4 digit suffixes. if we start the suffixes at 0000 and end with 9999 there are 10,000 different suffixes to choose. Multiply that with the 3 prefixes the answer is 3 * 10,000 = 30,000 different phone numbers