Official Solution:Barney is forming two large cubes, A and B, with small identical cubes. He used \(x\) small cubes to form cube A and \(y\) small cubes to form cube B. If \(x - y = 61\), then what is the value of \(x + y\)? A. \(64\)
B. \(125\)
C. \(169\)
D. \(189\)
E. \(216\)
Notice that since large cubes are made from small identical cubes, then both \(x\) and \(y\) must be perfect cubes (a perfect cube is a cube of an integer). So, \(x\) and \(y\) can be: 1, 8, 27, 64, 125, 216, ...
We are given that \(x-y=61\), so \(x\) and \(y\) are 125 and 64, respectively.
Thus, \(x+y=189\).
Note that while this question uses basic knowledge of lines and figures, it is actually not a Geometry question. There are 8 questions within GMAT Prep Focus Edition that use similar principles.
Here is one example.
Answer: D