Algebra question? (Probability and Statistics)?

I REALLY do not understand this. Please help!

How many different 5-digit codes are possible using a telephone keypad if the first digit cannot be 0 and no digit may be used more than once?

I know the answer is 27,216. I just don't know how to obtain that answer!

HELP! :)

five digit codes using 10 digits, first one can't be 0

think of choices per digit - they can't repeat, so one digit is used each time

9 * 9 * 8 * 7 * 6

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