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# What arithmetic should I memorize?

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Senior Manager
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What arithmetic should I memorize? [#permalink]

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25 Jun 2009, 21:39
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I find myself constantly using long division and multiplication for simple things like 13 x 11.

What are some good arithmetic calcs to memorize? I created a 20x20 multiplication table but this seems like a bit much right?

Perfect squares up to 100? $$\sqrt{3}$$? $$\sqrt{5}$$?

I am not really talking about formulas to memorize since you should definitely memorize things like nCr, nPr, sum of all #'s in an evenly spaced set, etc.
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Re: What arithmetic should I memorize? [#permalink]

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25 Jun 2009, 22:18
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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.
Attachments

Arithmetic Fractions and Percentages.doc [32.5 KiB]

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

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26 Jun 2009, 11:19
Thank you bb. I think I will just try to squeeze in as much as I can until test time.
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Re: What arithmetic should I memorize? [#permalink]

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10 Sep 2009, 13:53
Very gud doc....it will be helful for anyone who's taking GMAT...
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Re: What arithmetic should I memorize? [#permalink]

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22 Oct 2009, 00:59
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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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23 Oct 2009, 22:04
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@bb: Thanks
@enfinity: good technique but I guess one need time to get familiar and comfortable with it. Up to then the conventional way is better.
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Re: What arithmetic should I memorize? [#permalink]

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24 Oct 2009, 03:33
@eresh: what's the conventional way? memorizing?
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Re: What arithmetic should I memorize? [#permalink]

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24 Oct 2009, 21:25
enfinity wrote:
@eresh: what's the conventional way? memorizing?

Well...god have not gave me such kind of superbrain....I was taking about simple multiplication The one we were taught on schools....
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Re: What arithmetic should I memorize? [#permalink]

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31 Oct 2009, 09:56
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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.

It really saves time.
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Re: What arithmetic should I memorize? [#permalink]

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31 Oct 2009, 20:21
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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.

It really saves time.

And if it equals more than 10, add a 1 to the first digit

EG

68*11 -> 6+8=14 -> 6 14 8 -> 748
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Re: What arithmetic should I memorize? [#permalink]

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01 Nov 2009, 13:40
bb - Many thanks, this is very helpful.
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Re: What arithmetic should I memorize? [#permalink]

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03 Nov 2009, 01:28
Indeed very helpful. Thanks a lot.
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Re: What arithmetic should I memorize? [#permalink]

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18 Nov 2009, 21:28
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Good document. Should bump this up for ppl to download and memorize
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Re: What arithmetic should I memorize? [#permalink]

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07 Jan 2010, 14:26
Thanks specially the 11 multiplication helps
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Re: What arithmetic should I memorize? [#permalink]

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07 Jan 2010, 17:13
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Also, see this post for some good theory: math-number-theory-88376.html
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Re: What arithmetic should I memorize? [#permalink]

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09 Jan 2010, 20:05
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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
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Re: What arithmetic should I memorize? [#permalink]

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11 Feb 2010, 10:34
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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!
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Re: What arithmetic should I memorize? [#permalink]

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11 Feb 2010, 22:41
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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!
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Re: What arithmetic should I memorize? [#permalink]

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14 Feb 2010, 06:12
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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!

welldone dude,
just a small addition which i tried and will help to avoid confusion:

start writing the answer from right hand side in case if the addition of two no. exceeds 10, and add it to consecutive no. on left hand side. try out!!
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Re: What arithmetic should I memorize? [#permalink]

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14 Feb 2010, 06:21
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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!

hi,
i feel the process is bit complicated as it doesn't give the value of quotient, it just tells you whether no. is divisible by 8.
here is another tric, if the no. formed by last three digits is divisible by 8 then the whole no. is divisible by 8.

953360 is divisible by 8 since 360 is divisible by 8,
529418: not divisible as 418 is not divisible by 8.

plug in different values and try.
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Re: What arithmetic should I memorize?   [#permalink] 14 Feb 2010, 06:21

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