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Speed Math

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Intern
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Joined: 11 Jan 2012
Posts: 9
Location: India
Concentration: Operations, Finance
GMAT Date: 08-31-2012
GPA: 3.7
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Kudos [?]: 9 [1] , given: 11

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Speed Math [#permalink] New post 11 Oct 2013, 21:13
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Hey guys!

GMAT tests your knowledge as much as it does your ability to manage time. So, I hope this post helps you save a precious few seconds in trying to conquer the GMAT Quant section.

I believe that just by knowing how to multiply and divide by 2 (repeatedly), you can very QUICKLY handle the bigger numbers. Allow me to explain.

Multiplying by 5

say a number - 108

Now 108*5 can be written as (108/2)*10 = 54*10 = 540
338*5 = 169*10= 1690

Dividing by 5

Say a number - 162

162/5 = 162*2/10 = 324/10 = 32.4
412/5 = 824/10 = 82.4


Dividing by powers of 2, i.e 2,4,8,16,32 etc

Isnt it amazing that just by mastering dividing abd multiplying by 2 (which isnt difficult if you practice it a few times), you can do the same with much bigger numbers?

4 = 2^2 So you divide by 2, twice....say 38/4= 19/2 = 9.5
8 = 2^3 So you divide by 2, thrice....say 148/8 = 74/4 = 37/2 = 18.5
16 = 2^4, so you divide by 2, four times...548/16 = 274/8=137/4=68.5/2=34.25
the list goes on :)

Multiplying by powers of 2, i.e 2,4,8,16,32 etc

same goes with multiplication

4...multiply by 2 twice - 78*4= 156*2 = 312
8..112*8 = 224*4= 448*2= 896
16 = 130*16 = 260*8=520*4=1040*2=2080

Squaring any two digit number in a few seconds

Re-posting what I had posted a few minutes ago on another forum.

Here is a method to find squares of ANY 2 digit number in a matter of seconds :) For that you would need to remember squares of numbers upto 25, which I believe most of us know.

So, here goes:

say 44^2.

check difference with the number 25 - In this case, 44-25 = 19
Then check difference with 50 - In this case, 50-44=6. square this difference, i.e - 6^2 = 36

Now, join the 2 numbers...1936..there you go, that's your answer :)

A slightly more difficult number this time - Say 71

Step 1 ... 71-25 = 46
Step 2 ... 71 - 50 = 21...21 squared = 441...since we have to ensure we stick to 2 digits only, carryover the hundreds digit (4), by adding it to 46..so the result from step 1, now becomes 46+4 = 50

So, your answer is 5041 :)

Hope the above tricks help you save those precious seconds and minutes!
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Joined: 03 Nov 2009
Posts: 63
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Kudos [?]: 8 [0], given: 17

Re: Speed Math [#permalink] New post 13 Oct 2013, 12:16
Thanks sagiyer!

Some useful tips :-D

Best
Re: Speed Math   [#permalink] 13 Oct 2013, 12:16
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