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Please teach me fast math method for percents

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heel2891
 Post subject: Please teach me fast math method for percents
 Post Posted: Tue Jul 19, 2011 10:28 am 
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Students


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Can someone please explain in more detail than does the tutorial on here how to do fast math when you want to calculate this:

16 is what percent of 64? The method I do now is 16=x/100 * 64 then I solve for x, which can be cumbersome. I really want to understand the fast math method.

I understand how to do: what is 25% of 64. You just do 10% + 10% + 5%. So that would be 6.4 + 6.4 + 3.2 = 16.

I have that process down cold. But I don't understand how to do the reverse. If someone could explain it and give multiple examples that would be amazing!

Oh and if you have any other shortcuts/fast math techniques in other areas, please feel free to share. I'm a little slow and trying to improve my speed before test day.

Thanks,
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george.kourdin
 Post subject: Re: Please teach me fast math method for percents
 Post Posted: Thu Jul 21, 2011 12:08 am 
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http://www.amazon.com/How-Calculate-Qui ... 286&sr=8-1
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tim
 Post subject: Re: Please teach me fast math method for percents
 Post Posted: Fri Aug 12, 2011 11:09 pm 
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ManhattanGMAT Staff


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Location: Southwest Airlines, seat 21C
Heel, if you try it out you'll probably determine that the algebra is not all that cumbersome in the example you gave. I would like to suggest an alternative method for finding 25% of a number rather than adding 10% + 10% + 5%. Recall that 25% is 1/4, so just take 1/4 of 64. If you want, you can break that down to 1/2 of 1/2 of 64, which should be much easier. For this reason it's a good idea to memorize some of the more common percents and what they represent in terms of fractions..

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Tim Sanders
Manhattan GMAT Instructor
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