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2002 AMC 10B Problem 7

Problem 7 of 25EasierAlgebraNumber Theory

Let nn be a positive integer such that 12+13+17+1n\dfrac12 + \dfrac13 + \dfrac17 + \dfrac1n is an integer. Which of the following statements is not true?

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Solution

The sum 12+13+17+1n\dfrac12 + \dfrac13 + \dfrac17 + \dfrac1n is greater than 00 and less than 12+13+17+1<2,\dfrac12 + \dfrac13 + \dfrac17 + 1 \lt 2, so as an integer it must equal 1.1. Since 12+13+17=4142,\dfrac12 + \dfrac13 + \dfrac17 = \dfrac{41}{42}, we need 1n=142,\dfrac1n = \dfrac{1}{42}, so n=42.n = 42. Then 2,2, 3,3, 6,6, and 77 all divide 42,42, but n=42n = 42 is not greater than 84.84. So the false statement is n>84.n \gt 84. Thus, the correct answer is E.

More practice

Concepts: fraction · divisibility · bounding to limit cases

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