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# Machine A can complete a certain job in x hours. Machine B

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Manager
Joined: 17 Aug 2007
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Machine A can complete a certain job in x hours. Machine B [#permalink]

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23 Sep 2007, 23:16
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Machine A can complete a certain job in x hours. Machine B can complete the same job in y hours. If A and B work together at their respective rates to complete the job, which of the following represents the fraction of the job that B will not have to complete because of A's help?

(x - y)/(x + y)

x/(y – x)

(x + y)/xy

y/(x – y)

y/(x + y)

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GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5032

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Location: Singapore

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23 Sep 2007, 23:54
Machine A -> 1 job in x hours --> 1/x job per hour
Machine B -> 1 job in y hours --> 1/y job per hour

Toegther:
1/x + 1/y = (x+y)/xy job per hour --> 1 job in xy/(x+y) hours

In this amount of time, Machien B does x/(x+y) of the job. So it does not have to do 1 - x/(x+y) = y/(x+y) work.

Ans E

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CEO
Joined: 29 Mar 2007
Posts: 2553

Kudos [?]: 536 [0], given: 0

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24 Sep 2007, 01:05
carpeD wrote:
Machine A can complete a certain job in x hours. Machine B can complete the same job in y hours. If A and B work together at their respective rates to complete the job, which of the following represents the fraction of the job that B will not have to complete because of A's help?

(x - y)/(x + y)

x/(y – x)

(x + y)/xy

y/(x – y)

y/(x + y)

Just plug in some values. X=2hrs Y=4hrs.

So rates are 1/2 and 1/4. Combined they are 3/4. So it takes 4/3 hrs to complete the job working together. Or 1hr 20min.

Subtract 4hrs from 1hr 20 min. Difference is 2 2/3 or 2 hrs 40min.

So calculate how much work Y does in 2 2/3 hrs (8/3hrs). 1/4* 8/3 = 2/3 of the job.

So look for the choice that gives you this value.

A. 2-4/4+2 = -2/3 no.
try em all.

E: 4/2+4 = 2/3

This method is a bit slower, and its not completely full proof. Ex/ if you chose x=4 and y=2. Then A and E work. In this situation ud have to go back and calculate the difference of the time, then calculate how much work Y did. But in any case if u didn't know wut to do, u at least only have 2 choices left.

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Re: Machine A can complete a certain job in x hours. Machine B [#permalink]

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14 Oct 2016, 23:39
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: Machine A can complete a certain job in x hours. Machine B   [#permalink] 14 Oct 2016, 23:39
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