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Re: working together at their constant rates , A and B can fill [#permalink]
27 Dec 2012, 02:07

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smartmanav wrote:

Can't we do this question using unitary method ?

Hi Smartmanav,

I guess this cannot be done by Unitary method as we are given the Constant rate of A i.e 25L/min. Now does it tell you the time A will take alone to fill the tank. No, because we do not the volume/size of the tank.

That is why method suggested by Bunuel of Rate*Time =Work

Thanks Mridul _________________

“If you can't fly then run, if you can't run then walk, if you can't walk then crawl, but whatever you do you have to keep moving forward.”

Re: Data sufficiency work rate problem [#permalink]
15 Jul 2010, 16:18

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ksharma12 wrote:

15. working together at their constant rates , A and B can fill an empty tank to capacity in1/2 hr.what is the constant rate of pump B? 1) A's constant rate is 25LTS / min 2) the tanks capacity is 1200 lts.

Can someone solve this beyond just showing what is sufficient?

I know the answer is A but I need to know how to solve to fully grasp the concept of only needing the first statement.

Thank you

Question: If A and B can fill 2 tanks in 1 hour together, then how many tanks can B fill up in 1 hour alone?

I'm setting this up as ratios of "x tanks per 1 hour" : \(\frac{a}{1hr}+\frac{b}{1hr}= \frac{2}{1hr}\)

1) This does NOT give you "a" because you need to know the size of the tanks. As we stated above, the ratios above show how many TANKS per hour each can fill, not how many gallons or liters. NOT sufficient

2) This gives you the tank's capacity, which is valuable in combination with 1). However, just knowing the tank's volume isn't useful since we don't know the flow rate of A. NOT sufficient

Now that we have both the flow rate and the volume of the tank, we can easily find the value of "a" in the above equation. Both statements together are sufficient.

Are you sure the answer is A? I definitely think it is C. Where is this question from?

Re: Data sufficiency work rate problem [#permalink]
16 Jul 2010, 07:10

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Expert's post

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ksharma12 wrote:

15. working together at their constant rates , A and B can fill an empty tank to capacity in1/2 hr.what is the constant rate of pump B? 1) A's constant rate is 25LTS / min 2) the tanks capacity is 1200 lts.

Can someone solve this beyond just showing what is sufficient?

I know the answer is A but I need to know how to solve to fully grasp the concept of only needing the first statement.

Thank you

Answer to this question is C not A.

\(rate*time=job\).

We are told that \((A+B)*30=C\), where \(A\) is the rate of pump A in lts/min, \(B\) is the rate of pump B in lts/min and C is the capacity of the tank in liters.

Question: \(B=?\)

(1) \(A=25\) --> \((25+B)*30=C\) --> clearly insufficient (two unknowns), if \(C=1200\), then \(B=15\) but of \(C=1500\), then \(B=25\).

(2) \(C=1200\). \((A+B)*30=1200\) --> \(A+B=40\). Also insufficient.

(1)+(2) \(A=25\) and \(A+B=40\) --> \(B=15\). Sufficient.

Re: Working together at their constant rates, A and B can fill [#permalink]
29 Aug 2013, 17:13

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The standard formula for solving this problem is AB/A+B=1/2 hrs we need A to get to B, where A is the time taken by Pipe A to fill ENTIRE tank alone from options the time taken by Pipe A to fill ENTIRE tank alone=1200/25=48 minutes substitute in equation above to get value of B. SIMPLE!

Re: Working together at their constant rates, A and B can fill [#permalink]
14 Oct 2014, 02:22

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Re: Working together at their constant rates, A and B can fill [#permalink]
14 Oct 2014, 07:39

So let's put this into an equation. We know that (Arate+Brate)30min=T, if T= the tank's capacity.

(1) now we know that Arate=25lirer/min, so we can plug that into the equation and we get (25lit/min+Brate)30min=T

This is not sufficient, because there are still too many unkowns. For example, it T=900 liters then Brate would equal 5 liters/min. But if T=9,000 then Brate would equal 275 liters/min

(2) now we know that T= 1200 liters, plugging that into the equation we get (Arate+Brate)30min=1200 liters

This is also not sufficient, because there are a variety of different combinations of Arate and Brate that can fit this equation. For example, Arate could be 10 liters per min and Brate could be 30 liters per min OR vice versa.

Now lets try them both together by plugging (1) and (2) into the equation:

(25lit/min+Brate)30min=1200 liters

divide both sides by 30 min and we get: 25 liters/min+Brate=40liters/min

that means Brate equals 15 liters, so the answer is (C) both together are sufficient

In the actual GMAT, you would not need to go so far as to figure out exactly what B rate. That would be a waste of time. As soon as you know what you CAN solve it with both, which can be determined by the fact that there is only 1 unknown then you should select C and move on. _________________

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