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Speed Math

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Joined: 11 Jan 2012
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Speed Math [#permalink]

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New post 11 Oct 2013, 22:13
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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 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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Re: Speed Math [#permalink]

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New post 13 Oct 2013, 13:16
Thanks sagiyer!

Some useful tips :-D


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Re: Speed Math [#permalink]

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Re: Speed Math [#permalink]

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New post 29 Mar 2017, 21:58
Thanks! Nice tips. Can you suggest more shortcut tips that can help in quick calculations?

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Re: Speed Math [#permalink]

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New post 29 Mar 2017, 22:19
Well you can check out some videos on Vedic maths (a very quick and efficient way of doing quick maths).

For example, Squaring a number ending in 5
let's say 65^2
The last two digits would be 5^2= 25
The first digit/digits would be= 6.7(the next number)=42
Your answer is 4225

Similarly let's square 115 (Big number...Huh!!) But it's easy.....
Last two digits: 5^2=25
First digits/digits= 11.12= 132 (Multiplication with 11 is easy: Add 012+120=132)
The answer is 13225

Hope it helps:)

Help me make my explanation better by providing a logical feedback.

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Don't quit.............Do it.

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Re: Speed Math [#permalink]

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New post 29 Mar 2017, 22:45
There are a lot of tips or tricks you could use to calculate quickly. However, please be sure to understand the concept of each tip or trick you are going to apply. It's clear that the only way to master anything is to understand it.

For multiplying or dividing with big numbers, yes it's really hard and costs too much time. For me, there are just some tricks you could use, with medium-size numbers. For example, you could calculate 125/5, 23*2, 140/4,... But how about 12.25/13, 23.78*12, 43.55*1.12,...

Remember that, in GMAT, calculation skill won't be tested but logic skill. That's it, in actual GMAT, it's better you find the best solution rather than try to remember some tricks for rapid calculation.

Actual LSAT CR bank by Broall

How to solve quadratic equations - Factor quadratic equations
Factor table with sign: The useful tool to solve polynomial inequalities
Applying AM-GM inequality into finding extreme/absolute value

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Re: Speed Math   [#permalink] 29 Mar 2017, 22:45
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