This Problem is Work!

Stacey Koprince —  April 3, 2013 — 5 Comments

gmat pumpRaise your hand if you love rate and work questions. They’re awesome, right? They tend to be fairly long, and the set-up is pretty complex, plus we get to build a table before we dive into the equations!

Oh, wait no those are all reasons why we can’t stand these problems.

Give yourself approximately 2 minutes to try the below GMATPrep problem. When you’re done, take a look at it again and ask yourself, Is there a better way to do this thing?

* Pumps A, B, and C operate at their respective constant rates. Pumps A and B, operating simultaneously, can fill a certain tank in 6/5 hours; pumps A and C, operating simultaneously, can fill the tank in 3/2 hours; and pumps B and C, operating simultaneously, can fill the tank in 2 hours. How many hours does it take pumps A, B, and C, operating simultaneously, to fill the tank?

(A) 1/3

(B) 1/2

(C) 2/3

(D) 5/6

(E) 1

Have you got an answer? Pick one anyway. Pretend it’s the real test: you can’t keep going till you pick an answer.

(Aside: why am I insisting that you pick an answer? It doesn’t matter “ this isn’t the real test! Actually, it really does matter. If you can’t make yourself pick a random answer right now, then you’re going to hang on too long and waste time during the real test. Practice making yourself let go.)

Okay, back to our problem. Yuck. Not just two but three separate pumps, all presumably operating at different rates. Some pretty ugly numbers. This is not going to be fun.

Hmm. It seems like we need to figure out the individual rates of the three pumps. The three pieces of information would let us write three equations, and we’d have three variables, so we could use simultaneous equations rules to find the rates for all three pumps. Then we can add them up to find out how long they take to do the job working together.

Let’s see. First, let’s label each pumps rate according to the pump name. Pump A’s rate is A, pump B’s rate is B, and pump C’s rate is C.

Together, A and B fill the tank in 6/5 hours; that is A’s time and B’s time add up to 6/5 hours. Remember the standard work formula: RT = W. If they take 6/5 hours to do 1 job, then their combined rate is R(6/5) = 1, or R = 5/6. As a shortcut, know that if a machine takes a certain length of time to do 1 job, you can just take the reciprocal of that time to find the rate. In this case, 6/5 hours to do the job becomes a rate of 5/6 per hour.

Pump A’s rate and pump B’s rate, then, add up to a combined rate of 5/6, for our first equation:

(EQN1) A + B = 5/6

Do the same thing with the other two pieces of info to get two more equations:

(EQN2) A + C = 2/3

(EQN3) B + C = 1/2

All right! We’re getting somewhere. Now, our next step is to figure out how to solve for A, B, and C.

At this point, people will have used at least 30 seconds and probably closer to 60 “ and the really hard math is still to come. Who thinks they’re going to get through the rest of it in another minute? Okay, honestly, I probably would but I really don’t want to. : ) There’s got to be a better way!

What are we solving for again? We want to find the combined rate for all three pumps, or A + B + C. Instead of finding each one individually, can you think of a way to solve for that combination: A + B + C?

If you’re not sure, take a minute to think about it before you keep reading.

Okay, try this: add all three equations together at once. We’ll end up with 2A + 2B + 2C = (5/6) + (2/3) + (1/2). Start simplifying.

2A + 2B + 2C = (5/6) + (4/6) + (3/6)

2A + 2B + 2C = 12/6

A + B + C = 6/6 = 1

If all three work together, they’ll get the job done in 1 hour.

The correct answer is E.

What if you can’t get through the math? How could someone make a reasonable guess on this problem. Take a little time to try to figure something out and then come back here and keep reading.

If A and B do the job in 6/5 of an hour, but we knock out pump B and substitute C instead, suddenly it takes 1.5 hours, so C must be slower than B. Likewise, A and C take 1.5 hours, but replace A with B and the time jumps to 2 hours, so B must be slower than A. The rates then are in this order: C < B < A.

Further, pump C must be significantly slower than A. It takes A and B about an hour to do the job, while B and C take 2 hours. Pump B is the same, but A swaps out for C and the time doubles!

The two faster pumps, A and B, need a little over an hour to finish the job. If the slowest, C, is added, that’ll reduce the overall time. None of the answers is over 6/5 “ too bad. But keep thinking. If it takes the two faster ones a little over an hour to do the job, then adding the slowest pump isn’t going to speed things up all that much. For instance, the slowest pump won’t suddenly drop the entire job to only 1/3 of an hour! If that happened, then C would have to be the fastest pump.

Further, C has to be significantly slower than A, remember? So even 1/2 is probably too high. The answer is likely either C, D, or E. Pick one and move on.

Key Takeaways for Annoying Work Problems

(1) Sure, you can always set up those charts, write out all of the equations, and solve for each variable individually. But isn’t there an easier way?!? Yes, most of the time there is. Study these enough in advance that you learn a bunch of shortcuts.

(2) For instance (and this applies to any algebra problem, not just a Work problem), when they’re asking us to solve for a combination of variables (in this case, A + B + C), consider whether you can solve for the whole combo at once rather than finding each individual value. Maybe you can’t “ but at least think about it. J

(3) Don’t forget to study how to guess! Do you want to spend 4 minutes doing all of the annoying math on this one (with a high likelihood of making a mistake, because there’s so much math to do)? Or do you want to admit that you haven’t found the easier way to do it and take 2 minutes to guess and move on? In this case, you could probably even save a little bit of time and still give yourself a 1/3 chance of getting it right!

* GMATPrep questions courtesy of the Graduate Management Admissions Council. Usage of this question does not imply endorsement by GMAC.

Stacey Koprince

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Stacey Koprince is an Instructor and Trainer as well as the Director of Online Community for Manhattan Prep. She's also a management consultant who specializes in corporate strategy. She has been teaching various standardized tests for more than fifteen years and her entire teaching philosophy can be summed up in five words: teaching students how to think.

5 responses to This Problem is Work!

  1. shishir pandey April 4, 2013 at 3:52 am

    i guess the final equation A+B+C=6/6=1 is the rate of three machines….though it will come same if we reciprocal it, but my ques is that is it a certain rule that we always work with rate and not with time in these type of problem?????

  2. Hi Stacey,

    I’m a male Korean GMATTER and found your tips and instructions in the blog are very powerful.
    I have several questions to ask and would like to get some of your thoughtful insights.
    Would you please reply to my email?

    Thanks and look forward to hear from you soon.

  3. While in India normally these questions can be worked out in less than 15 seconds

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