how many different 7 digit telephone numbers begin with 231

how many different 7 digit telephone numbers begin with 231- and contain no 9s

Solution

The first three digits must be 231, so look at the last four digits.

The first of the last four digits can be any number except 9. This gives 9 possibilities.
Similarly, there are 9 possibilities for the next three digits also.

So the required total number of possible outcomes is:

9*9*9*9 = 6561