Question

How many 7-digit telephone numbers can be created if the first digit must be 8, the second must be 5, and the third must be 2 or 3?

Answer 1

The first and second numbers have only one option, the third has 2, and the final four have 10 so:

1*1*2*10*10*10*10

1*2*10^4

2*10^4

20000

Answer 2

If the first digit must be 8 and the second must be 5, we only have to focus on the last 5 digits. The third must be 2 or 3, so there are 2 choices for that. For the rest of the digits, there are 10 digits to choose from for each, so we have 2*10*10*10*10 which is 20000 different 7-digit numbers.