Probability : Counting Principle

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It's a counting problem. Follow the condition carefully and you won't miss it.

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A social security number has 9 digits. How many different S.S numbers are possible if:

a. repetition of digits is permitted
b. repetition of digits is not permitted
c. the first digit cannot be a 0 and repetition is not permitted

I need the layout(counting principle)
ex. 9 8 7 - 6 5 - 2 1 4 3
- - - - - - - - -
Thankyou

I think the answer for a= 1,000,000,000
for b= 3,628, 800 and for c= ...