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# reducing fractions or portions without making a mistake

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Manager
Joined: 24 Oct 2012
Posts: 65
WE: Information Technology (Computer Software)
reducing fractions or portions without making a mistake [#permalink]

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05 Oct 2014, 12:43
Hi
I am totally going wrong in doing simple math multiplication and division while dealing with large numbers. Either they are taking way too longer and ending up in a mistake.
For example question here, a-circular-mat-with-diameter-20-inches-is-placed-on-a-square-55871.html

100 pi / 24*24

I came till here but some where making a mistake in coming to approximate proportion this fraction is close to. Any suggestions in this regard are much appreciated.
Magoosh GMAT Instructor
Joined: 28 Dec 2011
Posts: 4680
Re: reducing fractions or portions without making a mistake [#permalink]

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05 Oct 2014, 20:12
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GMatAspirerCA wrote:
Hi
I am totally going wrong in doing simple math multiplication and division while dealing with large numbers. Either they are taking way too longer and ending up in a mistake.
For example question here, a-circular-mat-with-diameter-20-inches-is-placed-on-a-square-55871.html

100 pi / 24*24

I came till here but some where making a mistake in coming to approximate proportion this fraction is close to. Any suggestions in this regard are much appreciated.

Dear GMatAspirerCA,
I'm happy to help.

First of all, here are a couple blogs you may find germane:
http://magoosh.com/gmat/2012/fractions-on-the-gmat/
http://magoosh.com/gmat/2012/the-power- ... mat-quant/

First of all, you are correct that the proportion is $$\frac{100pi}{24*24}$$

We can cancel one factor of 4 in the numerator and denominator, which leaves us with: $$\frac{25pi}{6*24}$$

Now, here's what I notice: 25/24 is exceptionally close to one, and pi is about 3, which means that (pi)/6 is close to 1/2. So we have something close to 1 times something close to 1/2, which would have to be close to 1/2. Answer = (C).

Whenever you have to approximate with pi, for most purposes 3 is a close enough approximation. If we are being really detailed about it, in some of the harder questions we might have to use 22/7, which is even more accurate.

Mike
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Mike McGarry
Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)

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Joined: 09 Sep 2013
Posts: 13800
Re: reducing fractions or portions without making a mistake [#permalink]

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16 May 2017, 01:15
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: reducing fractions or portions without making a mistake   [#permalink] 16 May 2017, 01:15
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