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If Pool Y currently contains more water than Pool X, and if Pool X is currently filled to 2/7 of its capacity, what percent of the water currently in Pool Y needs to be transferred to Pool X if Pool X and Pool Y are to have equal volumes of water?

(1) If all the water currently in Pool Y were transferred to Pool X, Pool X would be filled to 6/7 of its capacity.

I came across this DS question on my last MgCAT, and while I do understand how to solve it, I fail to see why (according to the Mg explanation) we should consider both pools capacity as equal, when nothing in the text leads to that assumption… (or am I missing something here?)

Re: Can someone clear out this DS question for me plz. [#permalink]

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07 Dec 2010, 09:10

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itnas wrote:

Hi.

I came across this DS question on my last MgCAT, and while I do understand how to solve it, I fail to see why (according to the Mg explanation) we should consider both pools capacity as equal, when nothing in the text leads to that assumption… (or am I missing something here?)

Therefore I felt for E.

Thanks for the help.

If Pool Y currently contains more water than Pool X, and if Pool X is currently filled to 2/7 of its capacity, what percent of the water currently in Pool Y needs to be transferred to Pool X if Pool X and Pool Y are to have equal volumes of water?

(1) If all the water currently in Pool Y were transferred to Pool X, Pool X would be filled to 6/7 of its capacity.

(2) Pool X has a capacity of 14,000 gallons.

We are nowhere told that the capacities of pools X and Y are equal.

(1) If all the water currently in Pool Y were transferred to Pool X, Pool X would be filled to 6/7 of its capacity --> together pools X and Y contain 6/7 of the capacity of pool X. Now, in order pools X and Y to contain equal amount of water each pool should contain 3/7 of the capacity of pool X, thus from pool Y, which contains 4/7 (6/7-2/7=4/7) of the capacity of pool X we must transfer 1/7 of the capacity of pool X, which is 25% of the water currently in pool Y (1/7)/(4/7)=1/4. Sufficient.

Or consider the following: Let the capacity of pool X be 7 gallons. It's 2/7 full, thus there are 2 gallons of water. If all the water currently in Pool Y were transferred to Pool X, Pool X would be filled to 6/7 of its capacity and thus will contain 6 gallons of water, which means that there are now 6-2=4 liters of water in pool Y (it doesn't matter what capacity it has). In order both pools to contain equal amount of water each pool should contain 3 gallons of water, thus we should transfer 1 gallon from pool Y to pool X. 1 gallon is 1/4 of the water currently in pool Y.

(2) Pool X has a capacity of 14,000 gallons. No info about pool Y. Not sufficient.

Question: Let the capacity of X be C so, X= 2C/7 and Y>X If n is the quantity of water taken from Y and put into X Then, Y-n = X+n 2n = Y-X => n = (Y-X)/2 n is the quantity that needs to be transferred. The % will (n/Y) * 100 % (Note, that we already have one equation with X and C, and we just need one more involving Y and C, if we can get it out of any of the statements, it should be enough to solve) Now lets take a look at the statements :

(1) If all the water currently in Pool Y were transferred to Pool X, Pool X would be filled to 6/7 of its capacity.

Y+X = 6C/7 and we know that X=2C/7

Hence, Y=4C/7

n = (Y-X)/2 = C/7 Therefore, % of water = (C/7)/(4C/7) *100 =100/4 =25%

Sufficient.

(2) Pool X has a capacity of 14,000 gallons.

We need another relation in Y and C or Y and X to be able to solve, not sufficient.

Ans: A _________________

PS: Like my approach? Please Help me with some Kudos.

Re: If Pool Y currently contains more water than Pool X, and if [#permalink]

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13 Sep 2014, 22:23

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Re: If Pool Y currently contains more water than Pool X, and if [#permalink]

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15 Jan 2016, 03:21

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).

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