How many different telephone numbers are possible in city​ A?

Question

The local​ seven-digit telephone numbers in city A have $555$ as the first three digits. How many different telephone numbers are possible in city​ A?

Answer 1

There are $10$ possibilities ($0, 1, 2, 3, 4, 5, 6, 7, 8, $ and $9$) for the fourth digit,

$10$ for the fifth digit, $10$ for the sixth digit, and $10$ for the seventh digit -- $10^4$ altogether.

Answer 2

$10^4=10000$, because for the remaining $4$ digits, all $10$ options are possible.

Answer 3

There are seven total digits. The first 3 are already selected, so there is a choice for 4 digits. There are 10 possible choices for each of the other 4 digits, so there are a total of $10×10×10×10$ or $10^4$ choices