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2002 AMC 12B Problem 4

Problem 4 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

Since 12+13+17=4142,\dfrac12+\dfrac13+\dfrac17=\dfrac{41}{42}, the sum 4142+1n\dfrac{41}{42}+\dfrac1n lies strictly between 00 and 2,2, so it must equal 1.1. Then 1n=142,\dfrac1n=\dfrac1{42}, giving n=42.n=42. Now 2,2, 3,3, 6,6, and 77 all divide 42,42, but n>84n\gt84 is false. The untrue statement is n>84.n\gt84. Thus, the correct answer is E.

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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.