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2006 AMC 10A Problem 21

Problem 21 of 25HarderNumber TheoryCounting & Probability

How many four-digit positive integers have at least one digit that is a 22 or a 3?3?

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Solution

There are 90009000 four-digit integers. For those avoiding 22 and 3,3, the leading digit is one of {1,4,5,6,7,8,9}\{1, 4, 5, 6, 7, 8, 9\} (77 choices) and each remaining digit is one of {0,1,4,5,6,7,8,9}\{0, 1, 4, 5, 6, 7, 8, 9\} (88 choices): 783=3584.7 \cdot 8^3 = 3584. So 90003584=54169000 - 3584 = 5416 have at least one 22 or 3.3. Thus, the correct answer is E.

More practice

Concepts: complementary counting · digits

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.