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2024 AMC 12A Problem 6

Problem 6 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

To obtain a small positive sum, use two negative integers p,q-p,-q and one positive integer r,r, where pqr=60.pqr=60. Up to order, the positive factor triples of 6060 are (1,1,60),(1,2,30),(1,3,20),(1,4,15),(1,5,12),(1,6,10),(2,2,15),(2,3,10),(2,5,6),(3,4,5). \begin{gathered} (1,1,60),(1,2,30),\\ (1,3,20),(1,4,15),\\ (1,5,12),(1,6,10),\\ (2,2,15),(2,3,10),\\ (2,5,6),(3,4,5). \end{gathered} A positive value of rpqr-p-q is smallest when the largest factor is chosen as r.r. The positive values from the list are 58,27,16,10,6,3,11,5;58,27,16,10,6,3,11,5; the last two triples give negative values. Thus the least positive sum is 1016=3.10-1-6=3. (Three positive integers have sum at least 3,3, and three negative integers cannot have positive product.) Thus, the correct answer is B.

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Concepts: factor · optimization · casework

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