64 small identical cubes are used to form a large cube

[guerrero]

64 small identical cubes are used to form a large cube .How many more cubes are needed to add one top layer of small cube all over the surface of the large cube ?

a)64
b)128
c)152
d)216
e)256

OA C

[Brent@GMATPrepNow]

64 small identical cubes are used to form a large cube .How many more cubes are needed to add one top layer of small cube all over the surface of the large cube ?

a)64
b)128
c)152
d)216
e)256

OA C

64 cubes will create a 4x4x4 large cube.
If we add at top layer to all of the sides, we get a larger 6x6x6 cube
A 6x6x6 cube requires 216 cubes.

216 - 64 = 152, so the answer is C

Cheers,
Brent

[J N]

Poorly worded question.

If you only need to cover the existing surface area of 4x4x4 cube then need 96 more cubes.

[Brent@GMATPrepNow]

Poorly worded question.

If you only need to cover the existing surface area of 4x4x4 cube then need 96 more cubes.

I guess it comes down to whether the smaller cube's surface is covered if the edges are exposed. Given the potential for ambiguity, this question might not be GMAT-worthy.

Cheers
Brent

[J N]

too much ambiguity for sure. even the way you did it i would not have gotten it because i would say there need to be 216 more cubes to cover the original 64 ( I would not subtract the 64 from count) .

[vipulgoyal]

what about 5x5x5 - 4x4x4 if min required