A cylinder is being filled with sand weighing 200 kg per cub

Thanks guys.

I realized my screwup. My brain tends to turn to fog toward the end of study sessions.

Volume of a cylinder is pi(r)^2 x h.
r = 1/2, h =5
Therefore volume = 5pi/4....take this volume and multiply by 200. You get 250pi. (A)

What is quite interesting is that, while I love geometry problems I didn't even think that the diameter is double the radius.

Anyway, what I did was to find r using the circumference of the circle, 2πr. So, I basically said that the circumference of the circle the buttom of the cylinder) is 1, so 2πr=1 --> r= 1/2π.

Then I substituted this into πr^2*h, which is the capacity of the cylinder. This ended in a capacity of 5/4.

Finally, I did 5/4*200 = 2500. At this point I realised that 2πr=1 is not correct (I confused the word diameter with perimeter), but it still has the elements I needed and went with answer A, which is the correct one.

Even though I understand the correct solution, could you tell me what made my result this similar? I mean, in terms of "number properties". Because I guess there is a relationship I don't see...

Hi pacifist85,

The reason why you still got to the correct answer is more about co-incidence and "off-setting errors" than anything else.

The co-incidence is that whoever wrote the question chose to make the diameter 1 ft, so the radius = 1/2 ft.
When you tried to use the circumference....2r(pi) = 1....you ended up with 1/(2pi).

The offsetting errors take a bit more explanation....
1/(2pi) is close to 1/6, so you SHOULD have come up with V = (5)(pi)(1/2pi)^2 = 5/(4pi) = about 5/12.... which would have given you a final answer that should have been MUCH smaller (a little over 100 ft^3). But you ignored the pi in the denominator when you did the calculation, so you ended up with 5/4....which is what you get when you do the math correctly.

This is certainly a funny set of circumstances, but you have to 'weed out' these little mistakes from your 'process' - on Test Day, they can seriously hurt your performance.

GMAT assassins aren't born, they're made,
Rich

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