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Combined Work/Rates Problems Strategy?

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Combined Work/Rates Problems Strategy? [#permalink] New post 09 Oct 2009, 07:33
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I'm having a really difficult time with the combined work/rate problems in the quant section. Does anyone have any templates that they use? Tricks? Strategies? Any help/advice is much appreciated.

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Re: Combined Work/Rates Problems Strategy? [#permalink] New post 09 Oct 2009, 08:21
well here is how i go about it,

first of all i basically try to figure out how much percent of the total work can be done by A/B/C (etc) per time unit specified (hrs/days/weeks etc.)

so if we get this info, A can complete painting a wall in 12 days, then % of wall he can paint in one day would be 8.5%
and also if B can complete painting a wall in 20 days, then % of wall B can paint in one day would be 5%.

Now with this info we can do any combination in seconds :) .

like A paints for 2 days, then B joins in. How much more time will it take them now?

in 2 days A paints 8.5 x 2 = 17%.

A and B together paint 8.5 + 5 % of the wall per day = 13.5% of the wall per day

So % of wall left is 83%, per day can be painted 13.5% . no of days we need = 83/13.5 which will be close to one of the options you have. (very bad numbers to take for an example :P )

you can apply this to any kind of work related probs. I prefer dealing with %ages rather than fractions.

Just ask urself what can be done faster :- (1/15 + 3/5 + 1/20) or (6.66% + 60% + 5%)? :-D

Thats just my approach, i know there are better ones .
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Re: Combined Work/Rates Problems Strategy? [#permalink] New post 09 Oct 2009, 08:44
I def agree with you, percents are much better than fractions. I will try a few problems using your approach and get back to you. But I think this is a very good strategy.


What about the rate problems? Like for example, A starts running at 1pm and B starts running 10 minutes later, at what time will they catch up with eachother? Those rate ones also ALWAYS get me, traveling toward/away from eachother with a certain distance in between...any pointers?
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Re: Combined Work/Rates Problems Strategy? [#permalink] New post 09 Oct 2009, 08:55
Superprime wrote:
I def agree with you, percents are much better than fractions. I will try a few problems using you approach and get back to you. But I think this is a very good strategy.


What about the rate problems. Like for example, A starts running at 1pm and B starts running 10 minutes later, at what time will they catch up with eachother? Those rate ones also get me, traveling toward/away from eachother with a certain distance in between...any pointers?


well you just need to brush up with some relative speed probs.

remember these rules,

if two things are moving in opp directions then their total relative speed is = Speed of A + Speed of B

if two things are moving in the same direction then their total relative speed is = Speed of A - Speed of B

so if you have a prob like the one u stated, in that A has already been running for 10 mins.

So since we know A's speed we can get that distance covered.

When B starts he will cover the gap between him and A at their relative speed.

so time = distance/(Speed of A - Speed of B) [as they are moving in the same direction]

i dont think there is anything more to it. Just try a few probs to get some confidence in this. I guess that should be enough :)
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Re: Combined Work/Rates Problems Strategy? [#permalink] New post 09 Oct 2009, 09:12
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Another option for question type that you have problems with is to memorize an example.

I had an issue with overlapping sets questions, and what I had to do was memorize the example Kaplan gave. I memorized the exact words for the problem and then I memorized the solution and the final answer. Then, when I encountered these questions, I very quickly in minimal shape wrote out my problem and solution from memory (the equation basically) and use that model to plug the numbers in. These are very much template driven problems, so this approach worked perfectly for me - group 1, group 2, neither, and both and voila. if I ever have a doubt which group needs to be plugged in, I change the numbers in the new problem to match the old one and since I already know the answer for those numbers, all I have to do is match it. Fairly quickly I could do the whole plug in/out in my head, and I never had to worry about these questions again.
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Re: Combined Work/Rates Problems Strategy?   [#permalink] 09 Oct 2009, 09:12
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