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# What arithmetic should I memorize?

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Intern
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Re: What arithmetic should I memorize? [#permalink]

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21 Sep 2011, 23:29
Regarding Multiplication by 11, an easy way I use is to Multiply the number by 10 and then add the number to it.

i.e. A*11 = A*10 + A
e.g: 13*11 = 13*10 + 13 = 143
42*11 = 42*10 + 42 = 420 + 42 = 462

This is a generic method that can be used for many different multiplications:

A*21 = A*20 + A
A*19 = A*20 - A
A*110 = A*100 + A*10

Hope this helps somebody!

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Re: What arithmetic should I memorize? [#permalink]

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22 Sep 2011, 00:44
Nice nifty trick

enfinity wrote:
As far as common squares are concerned, I only remember the ones with 0 or 5 in the units digit. For the latter category, I use the following process:

For example: 65*65

1) Always write down 25 as this is always the last two digits of the result:
...25

2) Multiply (non-units digits) times (non-units digits + 1)
6 * (6+1) = 42

2c) Combine:
4225

This way I always have the important squares handy... very useful for estimations!

Any other math shortcuts? Anyone

Steve

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Re: What arithmetic should I memorize? [#permalink]

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22 Sep 2011, 00:52
gottabwise wrote:
Shelen wrote:
I'm new here, and try to go through all the topics - this site is a treasure! :)

Maybe not the right place to share my experience with "multiplication for simple things like 13 x 11", but anyway...if you need to multiply any two-digits number by 11, just sum those digits and put the result in between. For example,
13x11 -> 1+3=4 -> 143 is the result.
Or, 36x11 -> 3+6=9 -> the result is 36x11=396.

Thanks for the tips. I recently downloaded some math apps to learn shortcuts. I plan to use them minimially though given the geometry and other formulas I need to memorize.
It really saves time.

Posted from my mobile device

How about 46 X 11 = ? ; Just found out we cannot apply this rule to this so wondering
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Re: What arithmetic should I memorize? [#permalink]

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31 Oct 2011, 07:44
just add the digits and insert the result in between the number. If there is a carry over add it to the ten's digit of the number

So in this case 46*11 ; 4+6 = 10. Insert 0 in between 4 and 6, carry forward the 1, that gives us 506

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Re: What arithmetic should I memorize? [#permalink]

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19 Jan 2012, 20:58

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Re: What arithmetic should I memorize? [#permalink]

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08 Mar 2012, 00:46
Thank you for the document! Have already found it useful.

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Re: What arithmetic should I memorize? [#permalink]

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16 Mar 2012, 17:17
2
This post was
BOOKMARKED
I know this post is quite old, but I'm just discovering this forum (thankfully after only being 1 week into studying). I genuinely am surprised to see such great resources on here and I'm feeling more confident, even after just discovering gmatclub, that I'll get a good score.

I have a question about the downloadable sheet. I'm kind of confused with how it's ordered and why some of the items are included. Why are some fractions and some exponents on here, but others aren't? Is there a reason for these particular ones to be remembered? When a fraction or percentage is listed, should we be memorizing the decimal, percentage and fraction versions of each of them?

For the last three lines, why is it important to know the multiples of 12, 15 and 8 over other ones?

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Re: What arithmetic should I memorize? [#permalink]

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17 Mar 2012, 01:28
tkaelle wrote:
I know this post is quite old, but I'm just discovering this forum (thankfully after only being 1 week into studying). I genuinely am surprised to see such great resources on here and I'm feeling more confident, even after just discovering gmatclub, that I'll get a good score.

I have a question about the downloadable sheet. I'm kind of confused with how it's ordered and why some of the items are included. Why are some fractions and some exponents on here, but others aren't? Is there a reason for these particular ones to be remembered? When a fraction or percentage is listed, should we be memorizing the decimal, percentage and fraction versions of each of them?

For the last three lines, why is it important to know the multiples of 12, 15 and 8 over other ones?

The doc gives some arithmetics which one will frequently need while solving the GMAT questions. Though I agree that some useful staff is missing and presence of some other things is debatable.

As for the percentages, it's good to know fractional as well as decimal representation of some of them.
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Re: What arithmetic should I memorize? [#permalink]

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07 Oct 2012, 09:06

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Re: What arithmetic should I memorize? [#permalink]

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01 Dec 2012, 23:04
AtifS wrote:
Thanks for sharing the Doc bb...@Shelen & @jeckll...Thanks for the tip but I would like to add some more.

What if u need to multiply 3 digits or 4 didgits by 11...the procedure is same as u mentioned but it would be done like below

For 3 digits
133x11 --> 1 1+3 3+3 3=1463
And for 4 digits
1243x11 --> 1 1+2 2+4 4+3 3=13673

And for 5 digits
15453x11 --> 1 1+5 5+4 4+5 5+3 3=169983
and so on.
you can plug other numbers in and check it out.

Hope it helps!

This is awesome 'AtifS' ......!!! Great time saving...
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Re: What arithmetic should I memorize? [#permalink]

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01 Dec 2012, 23:22
AtifS wrote:
Guys! I just found another way of checking whether a number is divisible by 8 or not ( the rule is same but another approach or route ). It's for a number with more than two digits.

Let's take 1936
1) First of all check whether last two digits of the number are divisible by 4 or not.
For 1936, we do this way 36/4=9

2) If it is divisible by 4 then add the quotient to the 3rd last digit of the number and if the sum of them is divisible by 2 then the whole number is divisible by 8.

--> 9 (quotient)+ 9 ( 3rd digit from right)= 18, and -->18/2=9
So the whole number is divisible by 8.

Once you understand it and do a little practice, you'll find it easy and fast.
**You can try other numbers to see whether it is true or not
Hope it helps!

Well,this can be directly seen whether last three digits of any no. are divisible by 8.
EG : for same no : 1936, 936/8=117 => 1936 is divisible by 8

Let's take another no 7992, 992/8=124 => 7992 is divisible by 8.

So,you're getting your answer in 1 step and I think you don't need to memorize a lot for it
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Re: What arithmetic should I memorize? [#permalink]

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26 Dec 2012, 19:56
Good one imk... Kudos man...!

Hope it'll work out for all the relevant aspects for the series of even/odd Consecutive integers with Even no. of elements of course!

Btw, if you know any other shortcuts, could you please come up with those & help the Club..!
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Re: What arithmetic should I memorize? [#permalink]

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14 Apr 2013, 12:58
bb wrote:
Fantastic question

This file should give you an idea where you are lacking.
I think you should definitely know the squares from 0-10 and preferrably from 10 to 20 as well.

I am wondering if there is an error with the file that I downloaded. It just had random fractions and percentages, with no solutions to memorize! Why do I have the feeling I am missing something really obvious?

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Re: What arithmetic should I memorize? [#permalink]

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15 Apr 2013, 00:26
sns wrote:
bb wrote:
Fantastic question

This file should give you an idea where you are lacking.
I think you should definitely know the squares from 0-10 and preferrably from 10 to 20 as well.

I am wondering if there is an error with the file that I downloaded. It just had random fractions and percentages, with no solutions to memorize! Why do I have the feeling I am missing something really obvious?

Yes, answers are not given. You can get them yourself and memorize, though not everything in this doc is worth memorizing.
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Re: What arithmetic should I memorize? [#permalink]

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09 May 2013, 14:42
bb,
you are great!

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Re: What arithmetic should I memorize? [#permalink]

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09 May 2013, 16:15
sns wrote:
bb wrote:
Fantastic question

This file should give you an idea where you are lacking.
I think you should definitely know the squares from 0-10 and preferrably from 10 to 20 as well.

I am wondering if there is an error with the file that I downloaded. It just had random fractions and percentages, with no solutions to memorize! Why do I have the feeling I am missing something really obvious?

This is intended as a test - can you answer them right off the bat or quickly.... or are you wasting your time on these every day simple calculations.
This was my personal list and it may be slightly different for you. Bunuel seems to have his own - I'd love to see it
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Re: What arithmetic should I memorize? [#permalink]

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05 Dec 2013, 07:35
You know what, we have the same problem, I search it on the internet that's why I ended up here in you post.And Im very glad because I found the answer from the comments.

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Re: What arithmetic should I memorize? [#permalink]

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28 Dec 2013, 01:26
Well, I have been looking at some vedic maths and thought of putting it for everyone here..

Starting with multiplying.....We all know what is 40*50 ie. 2000. Now what s 55 *45....we need time correct...there is easier way for such question type where the unit digit is 5 and ten's digit are consecutive nos..For ex 1,2, or 3, 4 or 4,5 etc

Coming back to current problem.... 55*45
Step 1 : Keep as the last 2 nos of the product XX75
Step 2: Square the larger of the 2 ten's digit no.
In this case it is 5 and square is 25.
Step 3: Subtract 1 from 25 = 24
Ans will be 2475

Try any nos for eg. 25*35 will be 875 by same logic. For practice try these 95*85 or 65*75 or 135*125 (You can figure out the answers by calculator) and if you a problem, please post it here.

Similarly for squares of nos upto 60 can be easily calculated. ( I assume you know squares from 30-40 and obviously before 30) and using this method you can find squares of nos from 40 to 60 easily...Beyond this it is all about practice.

okay------->
Lets start...what is 53^2
Now we can always do it by making (50+3)^2 and simplify....Well I give another trick and you can figure out in less than 15 seconds

Step 1: 53 is 3 more than 50 and so the unit digit will be 3^2 which is 09
XX09

Step 2: Now 5^2 + unit digit will be the first 2 digits of this number. In this case it will be 28

2809

Consider another problem 58^2. Lets apply the same steps

1. 8^2 = 64 so we have XX64 because 58 is 8 more than 50
2. 5^2 +8= 33 so we have 3364

You can do it same way for nos greater than 60 also find it. For ex 65^2

1. 65 -----------> 65 is 15 more than 50 so 15^2 = 225.......keep the last 2 digits only xx25
2. Now 5^2 =25+15= 40
3. Add 2 from 225 and we get 4225

Try these nos 56^2 or 67^2
For nos less than 50...

What is the square of of 46^2

Step 1: Now 46 is 4 less than 50 so the nos will be XX16
Step 2: Now 5^ 2 is 25 and subtracting 4 from 25 we get 21.
So 46^2 will be 2116

What is 39^2 ??

Step 1: Square of 11(Cause 39 is 11 less than 50) is 121 and so Last 2 digits will 21 and so we have XX 21
Step 2: 5^2 -11 (Because 39 is 11 less than 50) we get 14 and add 1
We get 1521.....

try 43^2 or 48^2, 37^2

I hope you find them useful...Will put one on finding square of larger nos such as 101, 109, 119 etc.....

Happy gmating everyone
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Re: What arithmetic should I memorize? [#permalink]

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02 Jan 2014, 02:48
AtifS wrote:
Guys! I just found another way of checking whether a number is divisible by 8 or not ( the rule is same but another approach or route ). It's for a number with more than two digits.

Let's take 1936
1) First of all check whether last two digits of the number are divisible by 4 or not.
For 1936, we do this way 36/4=9

2) If it is divisible by 4 then add the quotient to the 3rd last digit of the number and if the sum of them is divisible by 2 then the whole number is divisible by 8.

--> 9 (quotient)+ 9 ( 3rd digit from right)= 18, and -->18/2=9
So the whole number is divisible by 8.

Once you understand it and do a little practice, you'll find it easy and fast.
**You can try other numbers to see whether it is true or not
Hope it helps!

if you just check whether the last 3 digits are divisible by 8 would become more easier rather then dividing it twice.

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What arithmetic should I memorize? [#permalink]

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14 Dec 2014, 03:09
hubahuba wrote:
I found this post a little too late but agree with bb that there are set number of things that everyone must know, out cold, by test day. This can save valuable time and give you opportunity to spend more time analyzing and solving questions rather than doing mundane computations.

Till 2 weeks back, I only knew tables up to to 10. The day and age we live in, I never felt the need. I either have excel or a calculator handy on my smart phone. These are habits one has to develop so I started putting together a Brain Dump sheet. I have roughly 30 print-outs on my desk, and first thing I do when I get to work is take 5 minutes to fill them out. Initially it will be difficult, but start by breaking them down into smaller sections till you have average proficiency.

You might want to add first 100 prime numbers to the list attached.

2 3 5 7
11 13 17 19
23 29
31 37
41 43 47
53 59
61 67
71 73 79
83 89
97

Learning aid:
2x, 5x and 8x are similar.
3x, 6x are similar
4x and 7x are similar.

bb wrote:
Fantastic question
This file should give you an idea where you are lacking.
I think you should definitely know the squares from 0-10 and preferrably from 10 to 20 as well.

A very good way to remember the prime numbers.
Tens digit of each 'similar' series differ from each other by 3.
The difference between highest and lowest prime number in each series is 6 (exception - 7x, where in 79-71=8)
Note: 4x and 7x are similar: Although the number of primes is 3 within the series, numbers with highlighted x are different.

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What arithmetic should I memorize?   [#permalink] 14 Dec 2014, 03:09

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