It is currently 20 Feb 2018, 01:44

Close

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Close

Request Expert Reply

Confirm Cancel

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

What arithmetic should I memorize?

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:

Hide Tags

Non-Human User
User avatar
Joined: 09 Sep 2013
Posts: 13823
Premium Member
Re: What arithmetic should I memorize? [#permalink]

Show Tags

New post 21 Dec 2015, 03:00
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________

GMAT Books | GMAT Club Tests | Best Prices on GMAT Courses | GMAT Mobile App | Math Resources | Verbal Resources

Intern
Intern
avatar
Joined: 17 May 2013
Posts: 20
Location: India
GMAT 1: 730 Q50 V38
GPA: 3.26
Reviews Badge
Re: What arithmetic should I memorize? [#permalink]

Show Tags

New post 07 Feb 2016, 17:39
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!


There's a pattern that numbers which are powers of 2 follow (2,4,8,16, and so on).

Here, if I were to check if 1936 is divisible by 8, I just need to know that 8=2^3.

Since 8 is 2 raised to the power of 3, for any number under consideration, I just need to check if the last 3 digits of the number are divisible by 8 or not (Here, 936=8*117). If the last 3 digits are divisible by 8, the entire number will be divisible by 8, irrespective of the number of digits in that number.

Another example:

Check if 1000032 is divisible by 16.

Now you know that 16=2^4, and the last four digits of the number quoted above (0032) are divisible by 16. Hence, the number is divisible by 16.

The same rule applies to 5. You can easily check divisibility by 5,25,125,625, etc. using the same rule!
Intern
Intern
avatar
Joined: 11 Sep 2016
Posts: 1
Re: What arithmetic should I memorize? [#permalink]

Show Tags

New post 12 Sep 2016, 09:33
Hi to everyone!

Want to share very useful tip on how to multiply integers that are both in 10<x<20 range.
Example 1:
13x15 = (13+5)x10+3x5 = 180+15 = 195
or
13x15 = (15+3)x10+3x5 = 180+15 = 195

Example 2:
18x19 = (18+9)x10+8x9 = 270+72 = 342

Such technic could save you 5-10 sec (depends on practice) per each computation
Intern
Intern
avatar
B
Joined: 10 Jun 2016
Posts: 12
Re: What arithmetic should I memorize? [#permalink]

Show Tags

New post 31 Mar 2017, 08:43
Thank you very much!
This is really helpful!
Director
Director
avatar
G
Joined: 02 Sep 2016
Posts: 786
Premium Member CAT Tests
What arithmetic should I memorize? [#permalink]

Show Tags

New post 04 Apr 2017, 00:21
2
This post was
BOOKMARKED
Though its important to understand the concept well but knowing few tricks (VEDIC MATHS) can speed up the process of solving questions under timed conditions.

Few tricks are:

(1) Squaring a number which ends in 5.
Square 25

Step 1: Write last two digits of the final answer: 5^5= 25
Step 2: multiple the first digit by the next bigger digit. 2*3=6

The answer is 625

(2) This can be used for elimination: if a number ends in 2,3,7, and 8 at unit's place, then that number won't be a perfect square.

(3) The common squares are:
2^2=4
3^2=9
4^2=16
5^2=25
6^2=36
7^2=49
8^2=64
9^2=81
10^2=100
11^2=121
12^2=144
13^2=169
14^2=196
15^2=225
16^2=256
17^2= 289
18^2= 324
19^2= 361
20^2= 200

Its better to know these squares but not necessary. Like I said knowing these can speed up the process of solving a question.

(4) Common cubes:
2^3= 8
3^3=27
4^3=64
5^3=125
6^3=216
7^3=343
8^3=512
9^3=729

(5) Square root of 0 is 0
Sq. root of 1 is 1
Sq. root of 2 is approx 1.41
Sq. root of 3 is approx.1.73

(6) Dividend= Divisor*Quotient+Remainder
0<=Remainder<Divisor

(7) Non-negative integers are 0,1,2,3,4,5,........
(8) 2 and 3 are the only consecutive prime nos.
And 2 is the only even prime no.

1 is not a prime no. as a prime no. has two factors 1 and itself. (1 has only one factor i.e. 1)

(9) Sum and product of three consecutive integers is always divisible by 3.
For example, 2,3,4
Product= 24 (3*8=24)
Sum= 9 (3*3=9)

(10) LCM(a,b)* GCD(a,b)= a*b
In LCM, we take the highest powers of the prime factors.
For example, LCM (2 and 4)
2= 2^1
4=2^2

We will take the higher power of 2 i.e. 2. The LCM is 4.

In GCD, we take the lowest power of the prime factors that are common.
For example, GCD (3,9)
3=3^1
9=3^2

GCD= 3

GCD is always less than or equal to the nos.

(11) Even nos. can be represented as 2n, 2n+2, 2n+4, and so on.
Odd nos. can be represented as 2n+1, 2n+3, 2n+5, and so on.

(12) 1 is the smallest divisor/factor of all the integers. Whereas 0 is the smallest multiple of every integer.

(13) Mean=Median of n consecutive integers.
For example, n, (n+1), (n+2), (n+3), and (n+4)
Sum= 5n+10
Mean= sum/n
=[5(n+2)]/5
=n+2

Median is the no. in the middle of the series.
If n is even, then take an average of the two middle terms.
If n is odd, take the middle term.

Here the median is (n+2).

(14) Always read the language of the question carefully as it can include important words such as inclusive, exclusive, distinct, positive integer, negative integer, factors, prime factors, and so on.
_________________

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

If you liked the post, HIT KUDOS !!

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

Intern
Intern
avatar
B
Joined: 30 Jul 2014
Posts: 6
Re: What arithmetic should I memorize? [#permalink]

Show Tags

New post 10 Dec 2017, 04:18
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


Hi!

Is this method applicable to all the squares? For example - how will you solve 89*89?
Expert Post
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 43828
Re: What arithmetic should I memorize? [#permalink]

Show Tags

New post 10 Dec 2017, 04:24
prateetchhatwal wrote:
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


Hi!

Is this method applicable to all the squares? For example - how will you solve 89*89?


This is only applicable for those which end with units digit of 5.
_________________

New to the Math Forum?
Please read this: Ultimate GMAT Quantitative Megathread | All You Need for Quant | PLEASE READ AND FOLLOW: 12 Rules for Posting!!!

Resources:
GMAT Math Book | Triangles | Polygons | Coordinate Geometry | Factorials | Circles | Number Theory | Remainders; 8. Overlapping Sets | PDF of Math Book; 10. Remainders | GMAT Prep Software Analysis | SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) | Tricky questions from previous years.

Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
Extra-hard Quant Tests with Brilliant Analytics

Re: What arithmetic should I memorize?   [#permalink] 10 Dec 2017, 04:24

Go to page   Previous    1   2   3   4   [ 67 posts ] 

Display posts from previous: Sort by

What arithmetic should I memorize?

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  


GMAT Club MBA Forum Home| About| Terms and Conditions| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.