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What arithmetic should I memorize?

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

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New post 16 Mar 2012, 17:17
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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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New post 17 Mar 2012, 01:28
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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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New post 03 Nov 2012, 01:09
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I would like to add one more trick or shortcut to find the square of a number... :roll:

Suppose you know square of 13 but you want to know whats the square of 14, then follow this..

13 square = 169

For 14 square, 13 square + 13 - { present number } + 14 - { the next number }

i.e 169 + 13 + 14 = 196 { which is 14 square }

Note - you may calculate 14 square by knowing 15 square also! :lol:

14 square = 15 square - 15 {present number} - 14 {previous number}

14 square = 225 - 15 -14

14 square = 196 :!:

Additionally if you club this technique with "finding squares of numbers ending with 5" then finding the square of big numbers id just a matter of few seconds.

This technique is very helpful when finding squares of big numbers, it will be mere addition or subtraction.

Consider Kudos if you like this post. :o :-D
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Re: What arithmetic should I memorize?  [#permalink]

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New post 29 Nov 2012, 10:10
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One useful tip (from vedic maths) -
multiplying numbers close to 100 or 1000 or so on
e.g. multiply 97 * 98
Take the base as 100 as both numbers are close to 100

Step 1) First, we multiply the offsets 2 and 3
We get 6.
Since our base is 100, which has 2 zeros, the product of offsets must also have 2 digits. Hence we write 6 as 06 and these are our last 2 digits.

Step 2) Now substract one of the integers ( 97 or 98) from the peer number's offset i.e. 97 - 2 or 98 -3. Either will give you 95.
These are our first 2 digits.

So the answer is 9506.
Hope this helps.
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Re: What arithmetic should I memorize?  [#permalink]

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New post 26 Dec 2012, 18:19
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I found a trick or a shortcut to find the sum of the first half of consecutive integers( STRAIGHT,EVEN AND ODD) given the sum of one half of the set. This works only when the total number of elements in the set is even. but definitely saves a few seconds.

Let me illustrate with examples:

1. CONSECUTIVE INTEGERS : 6 TO 15
The Sum of the greater 5 numbers in a set of 10 consecutive integers is 65. Find the sum of the first 5 numbers.

Short cut:
Step 1: Multiply the no.of elements in each half : in this case 5 each and the spacing between each number in the set. in this case 1. ie., 5*5*1 = 25
Step 2: If the given sum is that of the greater numbers in the set, then subtract '25' to get the sum of the lower 5 numbers, i.e, 65-25 = 40 is the Answer
or if the given sum is that of the lower 5 consecutive numbers, then add '25' to get the sum of the greater 5 numbers i.e., 40+25 = 65.

2.Lets try this with 6 consecutive EVEN integers:

Find the sum of lower half of the numbers in a set of 6 consecutive even integers if the sum of the latter half is 30.

Step 1: 3*3*2 ( remember each half has 3 elements and the spacing between the elements is 2 as they are even) = 18
step 2: the given sum - step 1= 30 - 18 = 12 ( the sum of the even integers 2,4 and 6) is the answer.


3.Lets try for the 16 consecutive integers from 8 to 23. given sum of the greater 8 numbers in the set = 156

step 1: 8*8*1=64
step 2: given sum - step 1 = 156-64= 92 (which is the sum of numbers starting 8 thru 15)

4. Now lets try 24 consecutive ODD integers:

Find the sum of the second half of the elements of a set when the first half sums up to 168. The set contains Consecutive Odd integers.

Step 1: 12*12*2 = 288 ( Odd numbers are spaced evenly)
Step 2: given sum + 168 = 288+168 = 456

Check it out : the numbers are 3 to 49 inclusive.


try more examples. But remember it works only on CONSECUTIVE INTEGERS WITH EVEN NUMBER OF ELEMENTS. And when the sum of lower half is given, you need to ADD the given sum to step 1 and when the sum of greater half is given u need to SUBTRACT step 1 from the given sum.

Hope this helps! :)
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Re: What arithmetic should I memorize?  [#permalink]

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New post 24 Jul 2014, 14:01
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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.

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New post 25 Jul 2014, 04:46
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Hi,

I've created an excel file I use to test myself on a daily basis to hopefully strengthen my basics while studying for the exam. It tests 20x20 multiplication and some of the the common powers, roots, fractions etc. that bb included in his doc.

Just thought I'd share incase there are others that might find it useful.

Cheers
Rupert
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Re: What arithmetic should I memorize? &nbs [#permalink] 31 Jul 2018, 09:56

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