What is the surface area of the cube ? : GMAT Data Sufficiency (DS)
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# What is the surface area of the cube ?

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Manager
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What is the surface area of the cube ? [#permalink]

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26 Jul 2011, 05:42
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5% (low)

Question Stats:

91% (01:43) correct 9% (00:00) wrong based on 32 sessions

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What is the surface area of the cube? (Regular cube)

(1) The magnitude of the volume of the cube is 10 times the magnitude of the surface area on one face of the cube
(2) The length of the diagonal of a face of the cube is 10Root(2) inches.

The official answer provided was D however I think that B should be the answer as the in the Unit is not provided hence we cannot tell what the surface area would be. Please comment.
[Reveal] Spoiler: OA
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26 Jul 2011, 07:27
surface area of cube=6(side)^2
volume of cube=(side)^3
1) The magnitude of the volume of the cube is 10 times the magnitude of the surface area on one face of the cube
volume=10(side)^2
(side)^3=10(side)^2
therefore side of cube=10
therefore surface area=6*10^2=600
Therefore 1 is sufficient

2) The length of the diagonal of a face of the cube is 10Root(2) inches.
digonal of face of cube = digonal of square=(side)*root(2)
therefore side=10
therefore surface area=600
therefore b is sufficient
so answer is D(EACH statement ALONE is sufficient.)
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Kaustubh

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26 Jul 2011, 08:10
mehtakaustubh wrote:
surface area of cube=6(side)^2
volume of cube=(side)^3
1) The magnitude of the volume of the cube is 10 times the magnitude of the surface area on one face of the cube
volume=10(side)^2
(side)^3=10(side)^2
therefore side of cube=10
therefore surface area=6*10^2=600
Therefore 1 is sufficient

2) The length of the diagonal of a face of the cube is 10Root(2) inches.
digonal of face of cube = digonal of square=(side)*root(2)
therefore side=10
therefore surface area=600
therefore b is sufficient
so answer is D(EACH statement ALONE is sufficient.)

but first choice doesn't tell us the unit. It could be anything. Won't it effect the answer choice?
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26 Jul 2011, 09:56
I would be surprised if the OG didn't specify units, but when it comes to other sources I wouldn't be surprised at all. Whatever the unit, it would have to be common to the surface area of one side and the volume of the cube, so (1) is sufficient regardless.

When using sources not produced by GMAC, you should focus on the mathematical content of the questions - not the details, though I admit this is an important detail
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28 Jul 2011, 22:42

If units are not provided in any question on the GMAT (and you will see questions where this is the case), assume them to be consistent. For example, in statement (1) here, assume the units of the volume to be m^3 (or cm^3 or mm^3) and the units for the surface area to be m^2 (or cm^2 or mm^2) as the case may be.

One some number properties questions, you will find that numbers are often not specified as being integers, whole numbers, natural numbers etc. For example, a sample DS question could be:

What is x?
(1) x^2 = 34
(2) x>0

In this case, nothing is said about x (whether it an integer, whole number etc). Make no assumptions about x, except that it is real (i.e. assume it can be fractional, negative, positive, zero).
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Re: Cube Surface Area   [#permalink] 28 Jul 2011, 22:42
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