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Bunuel
Official Solution:

A certain rectangular box A, has height of \(x\) units, length of \(y\) units and width of \(z\) units, where \(x\), \(y\), and \(z\) are positive integers. A second rectangular box B, has each of its dimensions - height, length and width, 50 percent smaller than that of rectangular box A. If the surface area of rectangular box B is 2.5 \(unit^2\), what is the volume of rectangular box A?

A. \(\frac{1}{4} \ unit^3\)
B. \(1 \ unit^3\)
C. \(2 \ unit^3\)
D. \(4 \ unit^3\)
E. \(10 \ unit^3\)


We need to find \(Volume_A=xyz\), while given that \(Surface \ area_B = 2*(\frac{x}{2}*\frac{y}{2} +\frac{x}{2}*\frac{z}{2}+\frac{y}{2}*\frac{z}{2})=2.5\).

Simplify \(2*(\frac{x}{2}*\frac{y}{2} +\frac{x}{2}*\frac{z}{2}+\frac{y}{2}*\frac{z}{2})=2.5\):

\(xy+xz+yz=5\)

Recall that we are given that \(x\), \(y\), and \(z\) are positive integers. 5 is small enough number to quickly get by testing numbers that \((x, y, z) =(2, 1, 1)\) in any order.

\(Volume_A=xyz=2\).

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: C
Can you explain this part - how did you get (2,1,1). If we add up these nos, the result will not be 5 instead it will be 4. Not sure about the solution from this part.
Thanks
When you substitute \((x, y, z) = (2, 1, 1)\) into \(xy + xz + yz = 5\), you get the correct equation: \(2*1+2*1+1*1=5\).
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Apart from hit and trial and other systematic approach anyone?
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Apart from hit and trial and other systematic approach anyone?

Check here: https://gmatclub.com/forum/a-certain-re ... 84983.html
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This is a great question that’s helpful for learning. geometry is no longer a part of GMAT FE
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ritik0212
This is a great question that’s helpful for learning. geometry is no longer a part of GMAT FE


Check these two topics:
  1. GMAT Syllabus for Focus Edition
  2. Geometry Tested on GMAT Focus?

While specific geometry knowledge is not tested on GMAT Focus, not everything involving shapes, volumes, or areas requires specialized geometry knowledge. The area of a square or rectangle, the volume of a cube or rectangular solid, and the Pythagorean theorem are not considered specific geometry knowledge by the GMAT and can still be tested. Moreover, a question can involve shapes but test another area, such as combinations or other topics. There are several questions involving these concepts in the GMAT Prep Focus mocks
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I did not quite understand the solution. Do we have any certain method where we do not have to assume the values of xyz?
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I did not quite understand the solution. Do we have any certain method where we do not have to assume the values of xyz?

You can check alternative solutions here: https://gmatclub.com/forum/a-certain-re ... 84983.html Hope it helps.
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