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m and n are both positive. Pump A pumps n liters of water per minute.

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m and n are both positive. Pump A pumps n liters of water per minute. [#permalink]

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New post 24 May 2015, 04:10
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  35% (medium)

Question Stats:

72% (01:31) correct 28% (01:31) wrong based on 236 sessions

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m and n are both positive. Pump A pumps n liters of water per minute, and pump B pumps m liters of water per minute. Is m < n ?

(1) x is the number of minutes it takes pump B to fill a 100 liter container alone, and 3x is the number of minutes it takes pump B to fill a 100 liter container while pump A pumps water out of that container.

(2) It takes (3x/2) minutes for pump A to fill a 100 liter container alone.

This is a Yes/No DS question. Answering a definite "Yes" or a definite "No" means Sufficient. If the answer is sometimes "Yes" and sometimes "No", it means Maybe which means Insufficient.

The issue is rates. n and m are the rates of the two pumps. the question asks whether m < n, so the real issue is whether A pumps more water per minute than B does.

Stat. (1): No need for complicated rate boxes here. The information tells us that pump B is faster than pump A, since the 100 liter container gets filled even though A pumps water out of it. This means that m is not less than n, but that's a definite "No", which is sufficient. Stat.(1)->S->AD.

Stat. (2): Since x is only given in statement (1), stat. (2) alone tells you nothing about pump B. Thus, there is no way to compare the rates of the two pipes, and Stat.(2)->IS->A.

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Re: m and n are both positive. Pump A pumps n liters of water per minute. [#permalink]

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New post 24 May 2015, 17:29
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reto wrote:
m and n are both positive. Pump A pumps n liters of water per minute, and pump B pumps m liters of water per minute. Is m < n ?

(1) x is the number of minutes it takes pump B to fill a 100 liter container alone, and 3x is the number of minutes it takes pump B to fill a 100 liter container while pump A pumps water out of that container.

(2) It takes (3x/2) minutes for pump A to fill a 100 liter container alone.

This is a Yes/No DS question. Answering a definite "Yes" or a definite "No" means Sufficient. If the answer is sometimes "Yes" and sometimes "No", it means Maybe which means Insufficient.

The issue is rates. n and m are the rates of the two pumps. the question asks whether m < n, so the real issue is whether A pumps more water per minute than B does.

Stat. (1): No need for complicated rate boxes here. The information tells us that pump B is faster than pump A, since the 100 liter container gets filled even though A pumps water out of it. This means that m is not less than n, but that's a definite "No", which is sufficient. Stat.(1)->S->AD.

Stat. (2): Since x is only given in statement (1), stat. (2) alone tells you nothing about pump B. Thus, there is no way to compare the rates of the two pipes, and Stat.(2)->IS->A.


That is a great analysis, and I want to add one bit of strategic advice on top. If you recognize that the issue is about knowing which pump rate between A and B is faster, then it is very apparent that both statements together are sufficient. Statement 1 says that pump B can fill the container in x minutes, and statement 2 says that it takes pump A longer to fill it (3x/2 minutes). Therefore we know that together they are clearly sufficient, because we can see that pump B is faster than pump A. But when a question has such an "obvious C" answer choice, then it is most likely actually either answer choice A or B. Here the second statement is definitely not going to be sufficient, because it is only discussing one of the rates, but the first statement has more information and is more complicated. Thus even if you did not know exactly why statement 1 is sufficient, you should be very suspicious on this problem that it is!

Also on problems like this one, don't get baited into doing a bunch of math. The question gives you the volume of the container, enabling you to find x, which you could then use to find pump A's rate and pump B's rate. But none of that is necessary to know which is faster; don't do more math on data sufficiency questions than you have to, or you will burn time.

I hope this helps!
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m and n are both positive. Pump A pumps n liters of water per minute. [#permalink]

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New post 22 Jul 2015, 11:47
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Here is my observation.

Statement 1 : Says that it takes pump B x minutes to fill 100 litre tank. And pump B 3x minutes to fill a 100 litre tank while pump A pumps water out.

Now rate of pump B is n litre per min and rate of pump A is m litre per min.

In order for pump B to fill the tank itself while pump A pumps water out it is necessary that n litre per min > m litre per min because if (n-m) per litre rate is not positive then the tank will never fill up. Hence n-m will be positive and therefore n>m.

Statement 2 : Clearly insufficient.

Answer : A
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Re: m and n are both positive. Pump A pumps n liters of water per minute. [#permalink]

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Re: m and n are both positive. Pump A pumps n liters of water per minute.   [#permalink] 23 May 2017, 08:33
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