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Each week we'll be posting several questions from The Official Guide For GMAT® Quantitative Review, 2ND Edition and then after couple of days we'll provide Official Answer (OA) to them along with a slution.

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Re: If 18 is 15 percent of 30 percent of a certain number, what [#permalink]

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14 Mar 2014, 11:20

Answer in 30 seconds by just glancing the answers.

As question states the 15% of 30% of x =18.

A)9 . Less than 18. Not an answer. B)36 . 30 % is less than 18. Not an answer . Alternatively 1/3 (close to 30%) is 12 < 18. So not an answer C)40 . 30 % is less than 18. Not an answer . Alternatively 1/3 (close to 30%) is 12 < 18. So not an answer D)81 . 30 % is less than 18. Not an answer . Alternatively 1/3 (close to 30%) is 27 and 1/6 (close to 15%) 4 < 18. So not an answer E)400 . E is answer no need to calculation.

Alternatively solution

Start putting the biggest number and get the result in first try 15% of 30% of 400 =18.

The math involved in this question can be 'set up' (and dealt with) in a variety of ways.

Here, if you're going to take a calculation-based approach, converting the percents to fractions would likely help you speed up (and avoid the last bit of math).

We're told that 18 is 15 percent of 30 percent of a certain number. We're asked for that number.

18 = (.15)(.3)(X)

18 = (3/20)(3/10)(X)

18 = (9/200)(X)

Since 9/200 is a relatively small fraction (it's far less than 10%), we know that 18 is LESS than 10% of X. Based on the given answer choices, there's only one number that is LARGE enough to fit that description....

Re: If 18 is 15 percent of 30 percent of a certain number, what [#permalink]

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25 Aug 2017, 21:19

In the Stem it is asked about 18 = 15 % of 30 % of a number, 15% of a number is 18 means the number should be greater than 100, only answer E having number greater than 100, so answer should be E
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Regards, Naveen email: nkmungila@gmail.com Please press kudos if you like this post

Re: If 18 is 15 percent of 30 percent of a certain number, what [#permalink]

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26 Aug 2017, 10:27

nkmungila wrote:

In the Stem it is asked about 18 = 15 % of 30 % of a number, 15% of a number is 18 means the number should be greater than 100, only answer E having number greater than 100, so answer should be E

but you must calculate the answer , if in case the answer choices are above 100 then your method will not work, so my suggestion is although here it is applicable but when there are comparable choices do by actual calculation.
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