Sherpa Prep Representative
Joined: 15 Jan 2018
Posts: 147
Given Kudos: 0
Re: GRE Math Challenge #72- A solid cubical block of wood
[#permalink]
26 Jan 2018, 17:43
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.