64 small identical cubes are used to form a large cube

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guerrero: Master | Next Rank: 500 Posts; Posts: 227; Joined: Sun Apr 08, 2012 4:53 amFollowed by: 3 members; Thanked: 12 times

by guerrero » Tue Apr 16, 2013 1:52 pm

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

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Source: Beat The GMAT — Problem Solving | Tue Apr 16, 2013 1:52 pm

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Tue Apr 16, 2013 2:03 pm

guerrero wrote: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

Brent Hanneson - Creator of GMATPrepNow.com

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J N: Senior | Next Rank: 100 Posts; Posts: 44; Joined: Tue Mar 12, 2013 2:27 pm; Thanked: 3 times

by J N » Tue Apr 16, 2013 2:53 pm

Poorly worded question.

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

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Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; GMAT Score:770

by Brent@GMATPrepNow » Wed Apr 17, 2013 5:33 am

J N wrote: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

Brent Hanneson - Creator of GMATPrepNow.com

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J N: Senior | Next Rank: 100 Posts; Posts: 44; Joined: Tue Mar 12, 2013 2:27 pm; Thanked: 3 times

by J N » Sat Apr 20, 2013 8:01 am

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) .

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vipulgoyal: Master | Next Rank: 500 Posts; Posts: 468; Joined: Mon Jul 25, 2011 10:20 pmFollowed by: 4 members; Thanked: 29 times

by vipulgoyal » Sun Apr 21, 2013 9:30 pm

what about 5x5x5 - 4x4x4 if min required

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