Ratio of surface area

[Cheese12]

What is the ratio of the surface area of a cube to the surface area of a rectangular solid identical to the cube in all ways except that its length has been doubled?

1/4

3/8

1/2

3/5

2

OA:D

Hi guys,

I tried to do the question this way.. im not sure where i am going wrong. Please help to correct me.

Surface area of the Cube :
side "s" = 1 ....therefore surface area = 6

Surface area of the Rectagular object:
w= 1
l=2 (double of s)
Surface area = 2 * 6 = 12

Ratio = 6/12 = 1/2 ....


Not sure where I am going wrong

[neelgandham]

Let the sides of the cube be 1 unit. Then the surface are of the cube say SA1 = 6*(side)*(side) = 6*1*1

Then the sides of the cuboid are 1,2,1(As the length has been doubled).
Surface area of cuboid Say SA2 = 2((l*b)+(b*w)+(w*l), where l,b,w are length, breadth and width respectively.
SA2 = 2((2*1)+(1*1)+(1*2)= 2(2+1+2) = 10

Ratio = SA1/SA2 = [spoiler]6/10 = 3/5 Option D[/spoiler]

[GMATGuruNY]

What is the ratio of the surface area of a cube to the surface area of a rectangular solid identical to the cube in all ways except that its length has been doubled?

1/4

3/8

1/2

3/5

2

OA:D

Hi guys,

I tried to do the question this way.. im not sure where i am going wrong. Please help to correct me.

Surface area of the Cube :
side "s" = 1 ....therefore surface area = 6

Surface area of the Rectagular object:
w= 1
l=2 (double of s)
Surface area = 2 * 6 = 12

Ratio = 6/12 = 1/2 ....


Not sure where I am going wrong

Surface area = the sums of the areas of all the faces.
In a cube, the area of each face = e², and there are 6 faces:
SA = 6e².
In a rectangular solid, the areas of the faces are lw, lh, and wh, and there are two of each type:
SA = 2(lw+lh+wh).

In the problem above:
Let e=1.
SA of the cube = 6(1²) = 6.

In the rectangular solid, since the length is doubled, l=2, w=1, and h=1.
SA = 2(2*1 + 2*1 + 1*1) = 10.

Cube/Solid = 6/10 = 3/5.

The correct answer is D.