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Several: Percent Shortcut and Absolute value inequation

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Several: Percent Shortcut and Absolute value inequation [#permalink] New post 27 Apr 2011, 07:50
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100% (02:06) correct 0% (00:00) wrong based on 13 sessions
Hi all,


Just wondering what is your guys reasonning to QUICKLY solve a percentage such as 15% of 35, 50 $

I am refering as example as a question like 139, OG 12th Edition, Problem Solving:


A meal cost $ 35.50 adn there was no tax. If the tip was more than 10 pc but less than 15pc of the price, then the total amount paid should be:

A. 40-42
B. 39-41
C. 38-40
D. 37-39
E. 36-37

Fin here under my reasonning, I would appreciate your comments.

10 pc of 35.50= kidstuff! = 35, 50 + (35,50/10)= 39, 05

mmmm let's have a look at the answer choices, B seems to be correct at 80% sure (only answer including 30 as the least value), would be nice if I could confirm it by finding the other value.

15 pc of 35.50 believe it or not, it takes me about 1 min to find out that it is about 40,8... I would appreciate to know how you guys make such a percentage quickly?

Even by making 1,15* 35,50 it takes time to calculate it toroughly.

- Those is the 138 th question out of the 230 PS in the OG, what's the level of such and the questions before sub 600? At what question does it starts to be above 600 ? Not really fair from GMAC to provide just 1/4 or 1/3 of 600+ questions

- I am looking for a great ressource to review some theory about inequation and absolute value, free if possible..?
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Re: Several: Percent Shortcut and Absolute value inequation [#permalink] New post 27 Apr 2011, 10:35
To find 15% of a number:

- first find 10% of the number
- cut that in half; that will give you 5% of the number
- add the 10% and 5% figures to get 15% of the number

So if you want to find 15% of $8.80, you would:

- find 10%, which is 88 cents
- find 5% by cutting that in half, to get 44 cents
- add 88 and 44 cents to get $1.32

Similarly, to find 15% of $35.50,

- 10% of $35.50 is $3.55
- 5% of $35.50 is half of that, so is $1.775 --> since the question asks for an estimate only, you should round this off to $1.80 or $1.75 to make the final step simpler
- if we round off to $1.80, then 15% is very close to $3.55+$1.80 = $5.35
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Re: Several: Percent Shortcut and Absolute value inequation [#permalink] New post 27 Apr 2011, 15:27
10%(35.5) = 3.55
15%(35.5) = 5.325

total amount could have been 35.5+3.55 and 35.5+5.325

=> could have been between 39.05 and 40.625
=> approximately between 39 and 41

Answer is B.
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Re: Several: Percent Shortcut and Absolute value inequation [#permalink] New post 27 Apr 2011, 17:36
Expert's post
Duke007 wrote:
Hi all,

- I am looking for a great ressource to review some theory about inequation and absolute value, free if possible..?


Check out our blog for some percentages, inequalities, modulus and other concepts:
http://www.veritasprep.com/blog/categor ... om/page/3/
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Re: Several: Percent Shortcut and Absolute value inequation [#permalink] New post 27 Apr 2011, 18:50
Thanks awfully :-)
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Re: Several: Percent Shortcut and Absolute value inequation [#permalink] New post 28 Apr 2011, 03:40
35.50 + 3.55 < x < 35.50 + 5.32

Answer - B
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Re: Several: Percent Shortcut and Absolute value inequation [#permalink] New post 28 Apr 2011, 21:13
why do u even calculate the upper limit?
u have only one answer with 39! u r done!

and if u want to be sure sure - u can do quickly:
10% is 3.55 so 5% is half - around 1.7
so 3.5+1.5 = (a little more than) 5.

done!

no need to calculate it.
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Re: Several: Percent Shortcut and Absolute value inequation   [#permalink] 28 Apr 2011, 21:13
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