This problem considers social security numbers, which is a sequnce of 9 digits,

Question

This problem considers social security numbers, which is a sequnce of 9 digits, such as 008583790.

(a). How many different social security numbers are there?

(b). How many different social security numbers are there, in which the digit 8 appears exactly twice (such

as 008583790)?

(c). How many different social security numbers have no digit appearing exactly twice? For example,

008583790 has the digit 8 appearing exactly twice, and 008583590 has both 8 and 5 appearing exactly twice.

On the other hand, 008583890 has no digit appearing exactly twice.

Explanation / Answer

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