2006 AMC 10A Problem 21

Below is the professionally curated solution for Problem 21 of the 2006 AMC 10A, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2006 AMC 10A solutions, or check the answer key.

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Concepts:complementary countingdigits

Difficulty rating: 1450

21.

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

24392439

40964096

49034903

49044904

54165416

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.

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