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jmc

counting and probability senior

Problem

The numbers , and have something in common: each is a -digit number beginning with that has exactly two identical digits. How many such numbers are there?
Solution
Suppose that the two identical digits are both . Since the thousands digit must be , only one of the other three digits can be . This means the possible forms for the number are Because the number must have exactly two identical digits, , , and . Hence, there are numbers of this form. Now suppose that the two identical digits are not . Reasoning similarly to before, we have the following possibilities: Again, , , and . There are numbers of this form. Thus the answer is .
Final answer
432