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A 500-gallon cistern is filled to half its capacity with water. At wha

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A 500-gallon cistern is filled to half its capacity with water. At wha [#permalink]

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New post 28 Mar 2018, 23:41
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A 500-gallon cistern is filled to half its capacity with water. At what rate, in gallons per minute, is the amount of water in the cistern increasing if both the inlet pipe and outlet pipe are open?

(1) The inlet pipe pumps water into the cistern at a rate of 7 gallons per 30 seconds.
(2) The outlet pipe pumps water out of the cistern at the rate of 12 gallons every 1 minute 30 seconds.
[Reveal] Spoiler: OA

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A 500-gallon cistern is filled to half its capacity with water. At wha [#permalink]

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New post Updated on: 29 Mar 2018, 05:38
Bunuel wrote:
A 500-gallon cistern is filled to half its capacity with water. At what rate, in gallons per minute, is the amount of water in the cistern increasing if both the inlet pipe and outlet pipe are open?

(1) The inlet pipe pumps water into the cistern at a rate of 7 gallons per 30 seconds.
(2) The outlet pipe pumps water out of the cistern at the rate of 12 gallons every 1 minute 30 seconds.


Since the Q. asks for the rate of water increase- we must know the inlet and outlet rates
1. Provides the inlet rate- nothing can be said whether the amount of water in the cistern would increase or decrease untill we know about the outlet rate
2. Provides the outlet rate- nothing can be said whether the amount of water in the cistern would increase or decrease untill we know about the inlet rate

Combining both would give us the answer(till now we do not know whether the increase would be positive or negative, but it is sure that we will get the answer)
{For those who still seek the answer,
Inlet rate- 14 gallons/min.
Outlet rate - 12/1.5 (1.5 is 3/2)
- 12*2/3 = 8 gallons/min.
Diff. - 6 gallons/min is the rate of increase}

Ans. C
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Originally posted by aggvipul on 28 Mar 2018, 23:54.
Last edited by aggvipul on 29 Mar 2018, 05:38, edited 1 time in total.
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Re: A 500-gallon cistern is filled to half its capacity with water. At wha [#permalink]

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New post 29 Mar 2018, 00:10
Bunuel wrote:
A 500-gallon cistern is filled to half its capacity with water. At what rate, in gallons per minute, is the amount of water in the cistern increasing if both the inlet pipe and outlet pipe are open?

(1) The inlet pipe pumps water into the cistern at a rate of 7 gallons per 30 seconds.
(2) The outlet pipe pumps water out of the cistern at the rate of 12 gallons every 1 minute 30 seconds.


To find the total rate, we'll need to know the rate of both the outlet and the inlet pipe or the change in volume over time.
We'll look for statements that give us this information, a Logical approach.

(1) what of the outlet pipe?
Insufficient.

(2) what of the inlet pipe?
Insufficient.

Combined:
We know have both the input rate and the output rate and can calculate the total rate of change of water in the cistern.
Sufficient.

(C) is our answer.
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Re: A 500-gallon cistern is filled to half its capacity with water. At wha [#permalink]

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New post 29 Mar 2018, 00:43
Hiii

Perfect approach to the Data sufficiency questions.
There is no need to solve for the values.
We just need to judge whether we can solve it or not using only the information provide vide the statement s and question stem.

[quite="DavidTutorexamPAL"]
Bunuel wrote:
A 500-gallon cistern is filled to half its capacity with water. At what rate, in gallons per minute, is the amount of water in the cistern increasing if both the inlet pipe and outlet pipe are open?

(1) The inlet pipe pumps water into the cistern at a rate of 7 gallons per 30 seconds.
(2) The outlet pipe pumps water out of the cistern at the rate of 12 gallons every 1 minute 30 seconds.


To find the total rate, we'll need to know the rate of both the outlet and the inlet pipe or the change in volume over time.
We'll look for statements that give us this information, a Logical approach.

(1) what of the outlet pipe?
Insufficient.

(2) what of the inlet pipe?
Insufficient.

Combined:
We know have both the input rate and the output rate and can calculate the total rate of change of water in the cistern.
Sufficient.

(C) is our answer.[/quote]
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Re: A 500-gallon cistern is filled to half its capacity with water. At wha   [#permalink] 29 Mar 2018, 00:43
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