Counting Techniques Question, Please Help!

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”?

I'm having some trouble with this question, can someone please explain how to do this? It would be very helpful, and for these type of questions how would you know if the elements can be repeated or not?

Answer 1

The first digit has 9 choices of 1-9, since 0 is not allowed. The second digit also has 9 choices since the first digit can't be reused. The third digit has 8 choices, since the first two can't be reused. The last seven digits each have choices 0-9 or 10 since digits can be reused.

N = 9×9x8x10x10×10×10 = 648 x 10⁴