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GRE Math Challenge #72- A solid cubical block of wood

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GRE Math Challenge #72- A solid cubical block of wood [#permalink] New post 03 May 2015, 01:05
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Question Stats:

63% (01:39) correct 36% (01:14) wrong based on 11 sessions
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A solid cubical block of wood has dimensions as shown in the figure, and the block is to be cut in half as indicated by the shaded region.

Quantity A: The total surface area of one of the resulting halves of the block
Quantity B: 36 square feet
• Quantity A is greater.
• Quantity B is greater.
• Both Quantities are Equal
• Cannot be determined

[Reveal] Spoiler:
Attachment:
Quant6.jpg
Quant6.jpg [ 8.56 KiB | Viewed 2269 times ]
[Reveal] Spoiler: OA

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Re: GRE Math Challenge #72- A solid cubical block of wood [#permalink] New post 17 Jan 2018, 00:24
Sandy, can you give solution, thanks!
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Re: GRE Math Challenge #72- A solid cubical block of wood [#permalink] New post 19 Jan 2018, 01:51
Quantity B is greater
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Re: GRE Math Challenge #72- A solid cubical block of wood [#permalink] New post 26 Jan 2018, 09:02
I thought it was B>A, but now I think A>B... Sandy, can you give the solution?
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Re: GRE Math Challenge #72- A solid cubical block of wood [#permalink] New post 26 Jan 2018, 15:05
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The result of cutting this cube is two prisms as shown below.

Attachment:
prism.png.jpg
prism.png.jpg [ 833.33 KiB | Viewed 589 times ]


Now the two triangles on the A and B form a square of side 3 feet. The base is a rectangle with one side 3 feet long and other \(3\sqrt{2}\) long (diagonal of a square is side times square root of 2).

So we have effectively 3 squares of 3 feet side length and one rectangle of sides 3 and \(3\sqrt{2}\).

Total area = 39.72.

Quantity A is greater.
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Re: GRE Math Challenge #72- A solid cubical block of wood [#permalink] New post 26 Jan 2018, 17:43
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Sandy's solution is correct, although I think he meant to answer A, not B. Here's how I would do it in a less math-intensive way.
Each side of the block is 9 sq ft. After it's been cut in half, we can just count sides of the block we've got. If we choose the block closest to us, then we've got the front side, the left side, and then the top and bottom sides. I'll ignore the cut through the middle for now. Since the top and bottom sides are each exactly half of a side, they can be thought of as on more side together. So since we've got 3 sides of 9 sq ft each, so far we've got 27 sq ft.
What about that last side? Well we know that quantity B is 36, so if the last side is 9, the quantities would be equal. However, we know it's bigger than 9 since its dimensions are going to be 3 by whatever the diagonal of the side is. I know the diagonal of a 3x3 square will be longer than 3, so the sides of the block must be bigger than 36 sq ft. Thus, A is the answer.

Now, we don't actually need to know the size of the last side to get the answer, but if you know your special triangles and square roots it would be pretty fast. Any square cut across its diagonal will form 2 right isosceles triangles with side ratios of 1-1-√2. So the diagonal of this particular square should be 3√2 and the dimensions of that central side would be 3x3√2. Since we know (or we should) that √2 is about 1.4, then we again know this last side will be bigger than 9.
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Re: GRE Math Challenge #72- A solid cubical block of wood [#permalink] New post 27 Jan 2018, 00:07
sandy wrote:
The result of cutting this cube is two prisms as shown below.

Attachment:
prism.png.jpg


Now the two triangles on the A and B form a square of side 3 feet. The base is a rectangle with one side 3 feet long and other \(3\sqrt{2}\) long (diagonal of a square is side times square root of 2).

So we have effectively 3 squares of 3 feet side length and one rectangle of sides 3 and \(3\sqrt{2}\).

Total area = 39.72.

Quantity B is greater.


I think you are wrong. The answer is A (39.72 square feet) which is bigger than B (36 square feet)
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Re: GRE Math Challenge #72- A solid cubical block of wood [#permalink] New post 28 Jan 2018, 03:53
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I meant A. Sorry fixed it.
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Re: GRE Math Challenge #72- A solid cubical block of wood   [#permalink] 28 Jan 2018, 03:53
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GRE Math Challenge #72- A solid cubical block of wood

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