2.0 Combinatorics

A bank requires its customers to create a 4-digit Personal Identification Number (PIN) using digits from 0 to 9.

a) How many different 4-digit PINs are possible if digits can be repeated?

b) How many different 4-digit PINs are possible if digits cannot be repeated?

c) How many different 4-digit PINs are possible if the first digit cannot be 0 and digits cannot be repeated?

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