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x percent of what number is equal to y?

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Math Revolution GMAT Instructor
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x percent of what number is equal to y?  [#permalink]

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New post 07 Sep 2016, 21:41
1
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A
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Difficulty:

  35% (medium)

Question Stats:

68% (01:25) correct 32% (01:35) wrong based on 232 sessions

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x percent of what number is equal to y?
1) x=10y
2) xy=40

*An answer will be posted in 2 days.

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Re: x percent of what number is equal to y?  [#permalink]

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New post 07 Sep 2016, 22:10
2
let the number be z, hence question asking x percent of what number is equal to y is x/100(z) = y
A) x=10y i.e. y=x/10
putting this value is the question equation => x/100(z) = x/10 i.e z=10 , sufficient

B) xy= 40 i.e. y=40/x => x/100(z) = 40/x => x^2(z) = 400 , can't be solved for z , not sufficient.

Answer is A
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Re: x percent of what number is equal to y?  [#permalink]

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New post 09 Sep 2016, 05:38
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1
MathRevolution wrote:
x percent of what number is equal to y?

1) x = 10y
2) xy = 40



Target question: x percent of what number is equal to y?
This is a good candidate for rephrasing the target question

x percent of what number is equal to y?
Let k = the number we're trying to determine

We get: "If x percent of k is equal to y, what is the value of k?
In other words, x/100 of k = y
Or....(x/100)(k) = y
To solve for k, multiply both sides by 100/x.
We get: k = 100y/x

Let's REPHRASE the target question.....
Target question: What is the value of 100y/x?
So, all we need to do now is find the value of x/y

Statement 1: x = 10y
Divide both sides by x to get: 1 = 10y/x
Divide both sides by 10 to get: 1/10 = y/x
PERFECT!
If y/x = 1/10, then 100y/x = 100(1/10) = 10
Since we can answer the REPHRASED target question with certainty, statement 1 is SUFFICIENT

Statement 2: xy = 40
This is NOT enough information to find the value of 100y/x
Consider these two conflicting cases:
Case a: x = 2 and y = 20, in which case 100y/x = 100(20)/(2) = 1000
Case b: x = 20 and y = 2, in which case 100y/x = 100(2)/(20) = 10
Since we cannot answer the REPHRASED target question with certainty, statement 2 is NOT SUFFICIENT

Answer =

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Re: x percent of what number is equal to y?  [#permalink]

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New post 09 Sep 2016, 07:15
Top Contributor
MathRevolution wrote:
x percent of what number is equal to y?
1) x=10y
2) xy=40

*An answer will be posted in 2 days.


One small note: The question should really include some information that tells us that x ≠ 0.
Otherwise, statement 1 is not sufficient.

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Brent
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Re: x percent of what number is equal to y?  [#permalink]

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New post 10 Sep 2016, 06:43
MathRevolution wrote:
x percent of what number is equal to y?
1) x=10y
2) xy=40

*An answer will be posted in 2 days.


question means
(x/100) a = y...find a, where a is the unknown number
x = ( 100/a ) y

1. gives us x = 10y => 10 = 100/a => a = 10....sufficient
2. xy = 40, we cannot modify it to look like original equation...not sufficient

Answer has to be A
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Re: x percent of what number is equal to y?  [#permalink]

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New post 10 Sep 2016, 20:31
If we modify the original condition and the question, we get x(1/100)(some)=y. So we have to find x/y. In case of condition 1), we get x/y=10. The condition is sufficient and the correct answer is A.

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Re: x percent of what number is equal to y?  [#permalink]

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New post 01 Sep 2018, 05:06
MathRevolution wrote:
x percent of what number is equal to y?
1) x=10y
2) xy=40

*An answer will be posted in 2 days.


This is how i solved

reword this
x/100*? =y

1) X= 10 Y ( Assume a value for Y )
LEts assume y =4 then 10y = 40
X=40

Now putting this back in the formulae
40/100*? =4
Solve to get 10 = sufficient

2) xy=40
Could be achived by multiple ossibilites
X= 20 and Y = 2
or X= 2 and Y = 20 in both cases we will get diffeirent answers to the question

NOt sufficnet

ANs = A
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Re: x percent of what number is equal to y? &nbs [#permalink] 01 Sep 2018, 05:06
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