[SOLVED] Counting Problem - just a quick question with this problem

Part a was this:

Q: How many positive integers between 1000 and 9999 inclusive are divisible by 9?

A: 9999 - 1000 + 1 = 9000

This question is part c.

Q: How many positive integers between 1000 and 9999 inclusive are distinct digit?

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part c I do not understand.

I have the answer as 9 x 9 x 8 x 7 = 4536. But I do not get why the 0 is not included in the 1st digit but the 0 is included in the 2nd?

This is what was given by my instructor:

1st digit is 9

2nd digit is 10 - 1 = 9

3rd is 10 - 2 = 8

4th is 10 -3 = 7

This is where he got 9 x 9 x 8 x 7.