# Resolve a GRE math problem

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Hello Evrybody!

Sorry if I post this in the wrong area!

I am studying like crazy for the GRE, and I didnt do math since... fiou.. 3 years??

I have 2 questions for you. I can't understand the explanations given by the practice test, i bold the things that doesn't make sense to me :

1- What is he surface area of a cube with volume = 512 united cubed ?

Answer: the volume of a cube is s^3 (s x s x s), so s^3 = 512 and so s=8. Surface area of 1 side = 8x8 = 64, so for the 6 sides = 384

2- I couldn't understant how they find z :

9 million = (z / 100) x 0,75 million

z = 1200 %

Thank you so much for your help!

And good luck for the people taking the GRE

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1- What is he surface area of a cube with volume = 512 united cubed ?

Answer: the volume of a cube is s^3 (s x s x s), so s^3 = 512 and so s=8. Surface area of 1 side = 8x8 = 64, so for the 6 sides = 384

S = the length of 1 side. Because the volume of a cube = width x length x height, and they are all the same, therefore s x s x s = s^3 = 512. Take the cube root of 512, you get 8.

2- I couldn't understant how they find z :

9 million = (z / 100) x 0,75 million

z = 1200 %

I believe you're missing the percentage sign in (z / 100). It should be (z / 100%). Rewrite the following on a piece of paper.

1) In the American way:

9 million = (z / 100%) x 0,75 million

(9 million / 0.75 million) = (z / 100%) x (0.75 million / 0.75 million)

12 = (z / 100%) x 1

12 = (z / 100%)

12 x 100% = (z / 100%) x 100%

1200% = z

Hence. z = 1200%

2) Using the Asian way:

9 million = (z / 100%) x 0,75 million

9/0.75 = z / 100%

12 * 100% = z

z = 1200%

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For the first one try guessing. You know that s x s x s = 512. Now, try 3 x 3 x 3 = 27, so you need a bigger number. Try 10 x 10 x 10 = 1000 so smaller, and so on. When you do a lot of those problems you start to get a feeling where the number should be. Like if it were 1567 then you know that it is more than 10, but not too much more.

9m=(z/100) x 0.75m

9/0.75=z/100

12=z/100

12 x 100=z

z=1200 (and you plug in the % sign at the end)

Hope that makes it clearr

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Thank you!! It was simple in fact! they didnt put the %, now it make so much sense! THANKS

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