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2018 AMC 10B Problem 4

Problem 4 of 25EasierAlgebraGeometry

A three-dimensional rectangular box with dimensions X,X, Y,Y, and ZZ has faces whose surface areas are 24,24, 24,24, 48,48, 48,48, 72,72, and 7272 square units. What is X+Y+Z?X + Y + Z?

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Solution

The three distinct face areas are the pairwise products XY=24,XY = 24, XZ=48,XZ = 48, YZ=72YZ = 72 in some order. Multiply all three: (XYZ)2=244872=82944,(XYZ)^2 = 24 \cdot 48 \cdot 72 = 82944, so XYZ=288.XYZ = 288. Now divide by each face area. We get Z=28824=12,Z = \frac{288}{24} = 12, Y=28848=6,Y = \frac{288}{48} = 6, and X=28872=4,X = \frac{288}{72} = 4, so X+Y+Z=22.X + Y + Z = 22. Therefore, the answer is B.

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Concepts: rectangular prism · surface area · system of equations

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