Counting Techniques Question

Question

How many seven-digit telephone numbers can be made if the first three digits must be different

and the number cannot start with “0”?

Please explain + steps, thank you.

Answer 1

1st #: 9 choices (no zero)

2nd #: 9 choices (1st number eliminated, zero is added)

3rd #: 8 choices (1st and 2nd number eliminated)

4th, 5th 6th, 7th each has 10 choices

(9)(9)(8)(10)(10)(10)(10) = ?