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Problems with Work/Rate questions

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Problems with Work/Rate questions [#permalink] New post 11 Feb 2011, 07:20
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Hi guys, Im studying second time for the gmat.. I got a 40q 22v split last time so I studied verbal a lot this time around.

In math I drastically need help on rate and work problems, whenever I see a question of these topics I skip it or guess, I don’t know what happens.. I an barely do them without a timer.

Can anyone suggest a way to understand these problems, I really want to take these topics on but I do not know where to start. I got yesterday a question about an elevator.. one guy was at the "x" floor, another guy was at " y" floor and both were closing up on each other at different rates and I had to see at which floor they would meet.
I really need help on these type of questions..

regards
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Re: Problems with Work/Rate questions [#permalink] New post 11 Feb 2011, 07:49
marijose wrote:
Hi guys, Im studying second time for the gmat.. I got a 40q 22v split last time so I studied verbal a lot this time around.

In math I drastically need help on rate and work problems, whenever I see a question of these topics I skip it or guess, I don’t know what happens.. I an barely do them without a timer.

Can anyone suggest a way to understand these problems, I really want to take these topics on but I do not know where to start. I got yesterday a question about an elevator.. one guy was at the "x" floor, another guy was at " y" floor and both were closing up on each other at different rates and I had to see at which floor they would meet.
I really need help on these type of questions..

regards


If you sincerely want to improve rate/work problem, try going through Word Translation book of Manhattan. They have addressed these questions very well. It will be totally worth buying and reading the book.
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Re: Problems with Work/Rate questions [#permalink] New post 11 Feb 2011, 07:59
Have you read any of the study guides on how to work those kinds of problems? MGMAT, Veritas, Princeton, etc? MGMAT in particular has a whole book on word translations.

The example you described is pretty easy--you can almost solve it in your head without writing out all the work/rate formulas. Let's say Person 1 starts on floor 8 going up at a rate of 5 floors per minute, and Person 2 starts on floor 2 going up at a rate of 7 floors per minute. That means that 2 is "gaining" on 1 at a rate of 2 floors per minute, so he would need 3 minutes to close the gap (3 times 2 equals 6, which is the initial difference in their floors).

You can check your math on that easily--after 3 minutes, Person 1 will be at floor 23 (5 times 3, plus 8), and Person 2 will be at floor 23 (7 times 3, plus 2).

If one elevator is going up and the other is going down, you just add their respective speeds--if one is going up at 5 floors/minute and one is going down at 7 floors/minute, then they're closing at a rate of 12 floors/minute.

Even if the question states it in terms of "x" and "y" you can still solve it by picking numbers and working it out as I did above, then plug those numbers into the answer choices to see which one works.
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Re: Problems with Work/Rate questions [#permalink] New post 11 Feb 2011, 08:03
I actually have the word translation book from MGMAT but havent started it .. I will look into it.

And this book adresses the work and rate problems??

I havent had any DIRECT studying into work or rates just the practice questions I come across in OG12 and reviews etc...

So you guys recommend I read the word translation book and I will be better prepared?

regards
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Re: Problems with Work/Rate questions [#permalink] New post 11 Feb 2011, 08:25
So you haven't done any direct studying of work/rate problems, but you're surprised that you're not good at them? I'm not trying to be rude, but think about how that sounds...I haven't played the piano in 15 years, but I don't expect to be good at it next time I sit down at one.

Yes, the MGMAT word translations book will help. OG doesn't teach you how to analyze and work the problems, it just gives you questions and answers.
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Re: Problems with Work/Rate questions [#permalink] New post 11 Feb 2011, 10:39
Im not surprized!

I dont want to be rude either but that is why I posted the question in the forum, I dont want to start studying something that isnt recommended by this comunity!.

Thanks for your comments Ill start with words translations in the comming days and see how it goes from there.

regards
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Re: Problems with Work/Rate questions [#permalink] New post 11 Feb 2011, 18:29
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marijose wrote:
Im not surprized!

I dont want to be rude either but that is why I posted the question in the forum, I dont want to start studying something that isnt recommended by this comunity!.

Thanks for your comments Ill start with words translations in the comming days and see how it goes from there.

regards


Work Rate problems are based on two and only two concepts:
1. Rates are additive.
2. Work done = Rate * Time (much like Distance = Speed * Time)


(Remember, Rate of work is just the speed of doing work)

If I paint half a wall in an hour and if you paint half a wall in an hour, if we both work together on a wall, we will finish the wall in an hour.
If my rate of work is 1/2 wall/hour and yours is 1/2 wall/hour, our total rate of work is 1/2 + 1/2 = 1 wall/hour (that is what we mean by 'rates are additive')

Now, if the question is: How many walls will we paint together in 8 hours?

Work = Rate*Time
Work = 1*8 = 8 walls

Use these two concepts and you will be able to solve most questions.

The basic questions of work rate are of the following form:
If A, working independently, completes a job in 10 hours and B, working independently, completes a job in 5 hours, how long will they take to complete the same job if they are working together?

Since A completes a job in 10 hours, his rate of work is 1/10th of the job per hour. B's rate of work is 1/5th of the job per hour.
Their combined rate of work would then be 1/10 + 1/5 = 3/10th of the job per hour.
As we said before, Work done = Rate * Time
1 = 3/10 * T (because 1 job has to be done)
or T = 10/3 hours.
This implies that A and B will together take 3.33 hours to do the job.
Note: Time taken when A and B work together will obviously be less than time taken by A or B when they are working independently.
Note: Time is not additive. In the question above, you obviously cannot say that A and B will together take 10+5 = 15 hrs to complete the job. Working together, they will take less time than either one of them working alone takes.

Try using these two concepts to solve your work-rate problems... Post specific questions if issues arise...
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Re: Problems with Work/Rate questions [#permalink] New post 16 Feb 2011, 06:49
Thanks a lot Karishma
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Re: Problems with Work/Rate questions [#permalink] New post 17 Feb 2011, 04:30
Karishma Nice one and Thanks.

Also , to avoid confusion we can take 100 or the LCM of the Numbers given in the Question as the 'base' on which we could use the Technique , taking the above example Into consideration - A takes 10 Hours to do some work - Let the work be production of some 100 Units

A takes 10 Hours to finish - 100 Units

B takes ONLY 5 Hours to finish the same 100 Units.

So the rate of A =100/10 = 10 Units per Hour.

And the Rate of B = 100/5 = 20 Units Per Hour.

Now In One Hour A makes 10 Units and B makes 20 Units = 10 +20 =30 unit / Hour (Both A & B)

Now If A& B together can make 30 Units per Hour , they make 100 Units in 100/30 = 3.33 Hours.

Best.
Re: Problems with Work/Rate questions   [#permalink] 17 Feb 2011, 04:30
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