How many different seven-digit telephone numbers are available if the only restriction is that the first digit can't be 0? Is there a formula for this to make it simpler? Then I'll solve this problem myself.

Answer 1

there are 9 ways to fill the first place

there are 10 ways to fill the second place, so far we have 9x10

there are 10 ways to fill the third place, so far we 9x10x10

etc.

Answer 2

9,000,000