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QuickER way to divide?

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Intern
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Joined: 03 Mar 2009
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QuickER way to divide? [#permalink] New post 26 Nov 2009, 23:03
I've been doing a few of the GMAT Club, but I use my own timer, rather than the one provided with the test. I do this so I can time myself to complete each question in under two minutes. I've been noticing that some of the quantitative questions require lots of division or simplifying fractions. A lot of the stuff is past my 13x13 times table, so I can't really figure it out so fast. Many times I have to do long division because the numbers are so obscure.

For instance, once I had to solve for x:

(40-x)16 = 550

or 1.86/12

It takes me longer than necessary to solve. I was wondering if you guys had any tips to speed up my arithmetic skills.

Thanks
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Joined: 17 Nov 2007
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Concentration: Entrepreneurship, Other
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Re: QuickER way to divide? [#permalink] New post 27 Nov 2009, 09:54
Expert's post
One of useful methods to speed up calculation is to use approximation.

Let's see:

1) 1.86/12
- if we use 10 instead of 12, we will decrease denominator by ~20%. Let's decrease numerator also by 20% (10% is 0.18, 20% is 0.36):
1.86/12 ~ 1.5/10 ~ 0.15

- or 1.86/12 ~ 1.8/12 ~ 6*0.3 / 6*2 ~ 0.15

2) (40-x)16 = 550

40-x = 550/16. I remember that 512 is 256*2 or 128*4 or 64*8 or 32*16. But 512 is too far from 550. We need add 2*16 to get closer number, 544.

So, 40-x ~ 32+2 (slightly larger) > x ~ 6 (slightly lesser)
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Posts: 49
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Kudos [?]: 39 [0], given: 3

Re: QuickER way to divide? [#permalink] New post 27 Nov 2009, 10:52
I like the first method of approximation a lot. I'm sort of confused about the second one you mentioned... can you please explain it in more detail? I don't understand why you ad 16*2 or the 32+2 part.
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Re: QuickER way to divide? [#permalink] New post 27 Nov 2009, 11:59
Expert's post
Nach0 wrote:
I like the first method of approximation a lot. I'm sort of confused about the second one you mentioned... can you please explain it in more detail? I don't understand why you ad 16*2 or the 32+2 part.


I know that 512 is divisible by 16 and I can easily find that it is 32. At the same time difference between 550 and 512 is ~ 40. So, I add 2*16 (32) to 512 and now I can say that 34 is very close to 550/16.
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Re: QuickER way to divide? [#permalink] New post 27 Nov 2009, 17:39
Ahh, ok, got it. Thanks!

Does anybody have the link to the file that has important numbers to remember?
Re: QuickER way to divide?   [#permalink] 27 Nov 2009, 17:39
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