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A 4 cm cube is cut into 1 cm cubes. What is the percentage

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A 4 cm cube is cut into 1 cm cubes. What is the percentage [#permalink] New post 06 May 2008, 20:44
A 4 cm cube is cut into 1 cm cubes. What is the percentage increase in the surface area after such cutting?


4%
166%
266%
300%
400%

Surface area of a cube = 6 * a ^2 = 6 * 4 * 4 = 24 * 4 = 80 + 16 = 90

I'm stuck here.

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Re: cube. [#permalink] New post 06 May 2008, 21:25
I guess the first thing we want to find out is how many new cubes will we see. To quickly calculate that divide the total volume of the big cube by the total volume of the small cube :

4^3 / 1^3 = 64 new cubes
surface area of big cube = 4^2 * 6 = 96 sqm
total surface area of new cubes = 64 * 1^2 * 6 = 384 sqm
% increase = (384 - 96)/96 * 100 = 300%

is this right ?
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Re: cube. [#permalink] New post 07 May 2008, 03:03
I agree with BSD's solution.

while searching I got the below formula for
finding different solutions for cubes..

If the cube of n*n*n cut into 1*1*1

Then

No of cubes no side painted = (n-2)*(n-2)*(n-2)
No of cubes exactly one side painted = (n-2)*(n-2)*6
No of cubes exactly two side painted = (n-2)*12
No of cubes exactly three side painted = 8

Using the above formula we have

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Re: cube. [#permalink] New post 07 May 2008, 04:59
alimad wrote:
A 4 cm cube is cut into 1 cm cubes. What is the percentage increase in the surface area after such cutting?


4%
166%
266%
300%
400%

Surface area of a cube = 6 * a ^2 = 6 * 4 * 4 = 24 * 4 = 80 + 16 = 90

I'm stuck here.


I get . 4*4*6 = surface area of original cube.

We get 64 small cubes. the surface area of these 1cm cubes are 6cm^3.

Now its just 6*64 = 384.

We find the % increase by: (384-96)/96 =3 or 300%.
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Re: cube. [#permalink] New post 07 May 2008, 05:33
Same Answer. 300%.
Re: cube.   [#permalink] 07 May 2008, 05:33
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