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2024 AMC 10A Problem 7

Problem 7 of 25EasierAlgebraNumber Theory

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+y2xy,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 60xyxy\frac{60}{xy}-x-y is 1016=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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Concepts: factor · optimization · casework

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