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2024 AMC 10A Problem 7

Problem 7 of 25EasierNumber TheoryProblem-Solving Techniques

The product of three integers is 60.60. What is the least possible positive sum of the three integers?

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Solution

A positive product comes from three positive integers or one positive and two negative integers. Three positive integers have sum at least 3.3. In the second case write the numbers as −x,−y,z,-x,-y,z, where x,y,zx,y,z are positive and xyz=60.xyz=60. A positive sum requires z>x+y≥2xy,z \gt x+y \ge 2\sqrt{xy}, so 60xy>2xy\frac{60}{xy} \gt 2\sqrt{xy} and hence xy<10.xy \lt 10. The possible products xyxy that divide 6060 are 1,2,3,4,5,6.1,2,3,4,5,6. Checking their factor pairs, the smallest positive value of 60xy−x−y\frac{60}{xy}-x-y is 10−1−6=3.10-1-6=3. Thus (−1)(−6)(10)=60,(-1)(-6)(10)=60, and no positive sum below 33 is possible. Therefore, the answer is B.
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