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2003 AMC 10A Problem 8

Problem 8 of 25EasierNumber TheoryCounting & Probability

What is the probability that a randomly drawn positive factor of 6060 is less than 7?7?

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Solution

The factors of 6060 are 1,1, 2,2, 3,3, 4,4, 5,5, 6,6, 10,10, 12,12, 15,15, 20,20, 30,30, and 60.60. Six of these twelve factors are less than 7,7, so the probability is 612=12.\dfrac{6}{12} = \dfrac{1}{2}. Thus, the correct answer is E.

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Concepts: factor · basic probability

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