Author 
Message 
TAGS:

Hide Tags

Senior Manager
Joined: 25 Mar 2009
Posts: 293

What arithmetic should I memorize? [#permalink]
Show Tags
25 Jun 2009, 21:39
23
This post received KUDOS
181
This post was BOOKMARKED
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.



Founder
Joined: 04 Dec 2002
Posts: 16807
Location: United States (WA)

Re: What arithmetic should I memorize? [#permalink]
Show Tags
25 Jun 2009, 22:18
75
This post received KUDOS
Expert's post
79
This post was BOOKMARKED
Fantastic question This file should give you an idea where you are lacking. I think you should definitely know the squares from 010 and preferrably from 10 to 20 as well.
_________________
Founder of GMAT Club
Just starting out with GMAT? Start here... or use our Daily Study Plan
Coauthor of the GMAT Club tests



Senior Manager
Joined: 25 Mar 2009
Posts: 293

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



Intern
Joined: 10 Sep 2009
Posts: 27

Re: What arithmetic should I memorize? [#permalink]
Show Tags
10 Sep 2009, 13:53
1
This post received KUDOS
Very gud doc....it will be helful for anyone who's taking GMAT...



Manager
Joined: 12 May 2009
Posts: 52

Re: What arithmetic should I memorize? [#permalink]
Show Tags
22 Oct 2009, 00:59
64
This post received KUDOS
55
This post was BOOKMARKED
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 (nonunits digits) times (nonunits 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



Manager
Joined: 05 Jul 2009
Posts: 172

Re: What arithmetic should I memorize? [#permalink]
Show Tags
23 Oct 2009, 22:04
1
This post received KUDOS
1
This post was BOOKMARKED
@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.



Manager
Joined: 12 May 2009
Posts: 52

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



Manager
Joined: 05 Jul 2009
Posts: 172

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



Intern
Joined: 24 Oct 2009
Posts: 24
Location: Russia
Schools: IESE, SDA Bocconi

Re: What arithmetic should I memorize? [#permalink]
Show Tags
31 Oct 2009, 09:56
60
This post received KUDOS
58
This post was BOOKMARKED
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 twodigits 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.



Intern
Joined: 31 Oct 2009
Posts: 34

Re: What arithmetic should I memorize? [#permalink]
Show Tags
31 Oct 2009, 20:21
26
This post received KUDOS
28
This post was BOOKMARKED
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 twodigits 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



Intern
Joined: 26 Oct 2009
Posts: 1

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



Manager
Joined: 05 Jul 2009
Posts: 172

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



Manager
Joined: 22 Sep 2009
Posts: 206
Location: Tokyo, Japan

Re: What arithmetic should I memorize? [#permalink]
Show Tags
18 Nov 2009, 21:28
1
This post received KUDOS
Good document. Should bump this up for ppl to download and memorize



Intern
Joined: 12 Oct 2009
Posts: 29
Location: I see you

Re: What arithmetic should I memorize? [#permalink]
Show Tags
07 Jan 2010, 14:26
Thanks specially the 11 multiplication helps
_________________
Be willing to fail. It's the price of greatness.



Founder
Joined: 04 Dec 2002
Posts: 16807
Location: United States (WA)

Re: What arithmetic should I memorize? [#permalink]
Show Tags
07 Jan 2010, 17:13
1
This post received KUDOS
Expert's post
3
This post was BOOKMARKED
Also, see this post for some good theory: mathnumbertheory88376.html
_________________
Founder of GMAT Club
Just starting out with GMAT? Start here... or use our Daily Study Plan
Coauthor of the GMAT Club tests



Manager
Joined: 24 Jul 2009
Posts: 181
Location: Anchorage, AK
Schools: Mellon, USC, MIT, UCLA, NSCU

Re: What arithmetic should I memorize? [#permalink]
Show Tags
09 Jan 2010, 20:05
1
This post received KUDOS
1
This post was BOOKMARKED
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 twodigits 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
_________________
Reward wisdom with kudos.



Senior Manager
Status: Not afraid of failures, disappointments, and falls.
Joined: 20 Jan 2010
Posts: 283
Concentration: Technology, Entrepreneurship
WE: Operations (Telecommunications)

Re: What arithmetic should I memorize? [#permalink]
Show Tags
11 Feb 2010, 10:34
43
This post received KUDOS
32
This post was BOOKMARKED
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!
_________________
"I choose to rise after every fall" Target=770 http://challengemba.blogspot.com Kudos??



Senior Manager
Status: Not afraid of failures, disappointments, and falls.
Joined: 20 Jan 2010
Posts: 283
Concentration: Technology, Entrepreneurship
WE: Operations (Telecommunications)

Re: What arithmetic should I memorize? [#permalink]
Show Tags
11 Feb 2010, 22:41
15
This post received KUDOS
21
This post was BOOKMARKED
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 19 36, we do this way 36/4= 92) 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!
_________________
"I choose to rise after every fall" Target=770 http://challengemba.blogspot.com Kudos??



Intern
Affiliations: isa.org
Joined: 26 Jan 2010
Posts: 21
Location: Mumbai, India.
Schools: ISB, India.

Re: What arithmetic should I memorize? [#permalink]
Show Tags
14 Feb 2010, 06:12
3
This post received KUDOS
2
This post was BOOKMARKED
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!!
_________________
"Great people don't do different things, they do things differently"



Intern
Affiliations: isa.org
Joined: 26 Jan 2010
Posts: 21
Location: Mumbai, India.
Schools: ISB, India.

Re: What arithmetic should I memorize? [#permalink]
Show Tags
14 Feb 2010, 06:21
3
This post received KUDOS
5
This post was BOOKMARKED
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.
_________________
"Great people don't do different things, they do things differently"




Re: What arithmetic should I memorize?
[#permalink]
14 Feb 2010, 06:21



Go to page
1 2 3 4
Next
[ 66 posts ]



