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Not very sure but just thought of the following way:-

A's rate is 1/x
B's rate is 1/y.

when they work together, then work is done in time xy/(x+y).
In this time, work done by b is x/(x+y), by a is y/(x+y) and total work is 1. So the work which b did not do that is the work done by a should be represented as the fraction of total work
a's work/total work = [y/x+y] / 1 = y/(x+y)

Option E
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Plugging in numbers,
Let A rate = 3
B rate = 2
To finish a work of 10,
B would work 5 hours to finish it, but only 4 hours with help of A. So the fraction left is 2/5

Plug in numbers
y/(x+y) = 2/5


Hence E

Hope that helps
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Another approach is algebraic.

Machine A completes a job in x hours so the rate is 1/x
Machine B complets a job in y hours so the rate is 1/y
Collectively 1/x + 1/y = x + y /(xy) for both working together

To get the fraction of what B doesn't have to complete because of A's help is simply to get the fraction of A's workrate when they are both working together


Hence (1/x)/(x+y)/xy = (1/x * xy)/(x+y) = y/(x+y)
Hope that helps again!
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job completed in 1 hour when A and B work together at their respective rates is:
1/x + 1/y = (x+y)/xy

to complete job total time taken = xy/(x+y)

in this time job completed by B= 1/y * xy/(x+y) = x/(x+y)

rest is done by A = 1- X/(x+y)=y/(x+y)
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Take an example to understand it.
A takes 2 hrs (= x) to complete a work alone and B takes 2 hrs (= y) to complete a work alone. Together, they will complete the work in 1 hr = [xy/(x+y) = 2*2/(2+2)]
So A does 1/2 of the work in 1 hr (A's rate of work) and B does 1/2 of the work in 1 hr (B's rate of work).

So when they were working together, each needed to work for only 1 hr.
How much work was done by A in that one hr? 1*(1/2) = (1/2)
So because of A's help, B doesn't need to do half of the work i.e. work done by A.
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My solution might be very simple:

If we pick numbers like x = 3hrs and y = 4hrs then it takes 7hrs to complete two units for both of them. (We can use two units because we are looking for fractional work, not a total quantity #). If it takes 7hrs between the two of them then Ma does 3/7 or the work and Mb does 4/7 of the work. This means that Mb is not doing 3/7 of the work and you can plug in your answer choices to find out which one gives you x=3, y=4 -> 3/7
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Take an example to understand it.
A takes 2 hrs (= x) to complete a work alone and B takes 2 hrs (= y) to complete a work alone. Together, they will complete the work in 1 hr = [xy/(x+y) = 2*2/(2+2)]
So A does 1/2 of the work in 1 hr (A's rate of work) and B does 1/2 of the work in 1 hr (B's rate of work).

So when they were working together, each needed to work for only 1 hr.
How much work was done by A in that one hr? 1*(1/2) = (1/2)
So because of A's help, B doesn't need to do half of the work i.e. work done by A.

This explanation is vague since you picked 1/2. We cannot see how it works because it does not matter what you do with x and y it's around 1/2.
I see that formula is absolutely correct but don't understand why if the value of 1/x is 2/6 and 1/y is 3/6, the portion which y should not do comes out as 3/5 not 2/5.
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\(\frac{1}{x}+\frac{1}{y}=W\)

Combining their efforts they could finish: \(W=\frac{x+y}{xy}\)

This means it will take them t to complete the job.

\(\frac{x+y}{xy}(t)=1==>t=\frac{xy}{x+y}\)

The job done by x will be the job that y doesn't have to worry about.

\(W=\frac{1}{x}(xy/x+y)==>W=\frac{y}{x+y}\)
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felixjkz

This explanation is vague since you picked 1/2. We cannot see how it works because it does not matter what you do with x and y it's around 1/2.
I see that formula is absolutely correct but don't understand why if the value of 1/x is 2/6 and 1/y is 3/6, the portion which y should not do comes out as 3/5 not 2/5.

The explanation was meant for saswani who had trouble understanding "Then why do we multiply the rate of only A with the total time?"
It doesn't matter what the numbers are - as long as they are easy to work with - you can make out what's going on.

As for your question "why if the value of 1/x is 2/6 and 1/y is 3/6, the portion which y should not do comes out as 3/5 not 2/5."

If x = 3 and y = 2,
A does 1/3rd work in 1 hr and B does 1/2 work in 1 hr.
When they work together, they complete the work in 1/(1/2 + 1/3) = 6/5 hrs

In 6/5 hrs, A does (1/3)*(6/5) = 2/5 work
and B does (1/2)*(6/5) = 3/5 work

So B does NOT need to do 2/5 of the work. Answer still (E).
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Can someone please explain the formula xy / x+y to state the work done together. I thought I had a well understanding of this concept, however when I saw this question, I lost it completely. Thanks in advance
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Can someone please explain the formula xy / x+y to state the work done together. I thought I had a well understanding of this concept, however when I saw this question, I lost it completely. Thanks in advance

Check here: two-consultants-can-type-up-a-report-126155.html#p1030079 and here: work-word-problems-made-easy-87357.html

Hope it helps.
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.......................... Rate ..................... Time ................. Work Done

Machine A ............... \(\frac{1}{x}\) ...................... x ........................ 1 (Assumption)

Machine B ................ \(\frac{1}{y}\) ....................... y ....................... 1

Combined rate ............ \(\frac{1}{x} + \frac{1}{y} = \frac{x+y}{xy}\) ............. \(\frac{xy}{x+y}\) ................... 1

Work done due to A's help = Rate of A * Time required for the combined

\(= \frac{1}{x} * \frac{xy}{x+y} = \frac{y}{x+y}\)

This will be the benefit for B

Answer = E
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I used the following approach but was not able to proceed, could somebody tell me how/whether we can solve further to get the answer
A = 2 hrs (x), B = 4 hrs(y).
A+B = 4/3 hrs.

If B alone is working, it takes 4 hrs. But both working, B has to work only for 4/3 hours.
So B does not work for 4 hrs - 4/3 hrs = 8/3 hours.

I then thought that B did not have to work for 8/3 hours since A pitched in.
So 8/3 hours multiplied by A's rate (1/2)...gives 4/3..

But none of the options give this value. Any reason this approach does not work?...could somebody pls help.
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Success2015
I used the following approach but was not able to proceed, could somebody tell me how/whether we can solve further to get the answer
A = 2 hrs (x), B = 4 hrs(y).
A+B = 4/3 hrs.

If B alone is working, it takes 4 hrs. But both working, B has to work only for 4/3 hours.
So B does not work for 4 hrs - 4/3 hrs = 8/3 hours.

I then thought that B did not have to work for 8/3 hours since A pitched in.
So 8/3 hours multiplied by A's rate (1/2)...gives 4/3..

But none of the options give this value. Any reason this approach does not work?...could somebody pls help.

B did not have to work for 8/3 hrs because A pitched in - great.
So what was the work that B would have done in this 8/3 hrs? Work = Rate*Time = (1/4)*(8/3) = 2/3 (it has to be B's rate)
Option (E) gives you 2/3.
So B did not have to do 2/3rd of the work.
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The earlier posters have demonstrated the two methods (Algebraic and Input-Output) for solving a question type I call Variables in the Answer Choices.

If you'd like more information on these approaches, we have some free videos:
- Variables in the Answer Choices - https://www.gmatprepnow.com/module/gmat- ... /video/933
- Tips for the Algebraic Approach - https://www.gmatprepnow.com/module/gmat- ... /video/934
- Tips for the Input-Output Approach - https://www.gmatprepnow.com/module/gmat- ... /video/935

Cheers,
Brent
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enigma123
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?

A) (x – y)/(x + y)
B) x/(y – x)
C) (x + y)/(xy)
D) y/(x – y)
E) y/(x + y)

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I'll solve this using a table with the smart numbers: x= 6 hours for Machine A, y= 4 hours for Machine B, and total work of 12 units.



Information Machine AMachine BTogether
Total work12 units12 units12 units
Time to complete alone6 hours4 hours?
Rate (units/hour)12 ÷ 6 = 212 ÷ 4 = 3
Time working together12 ÷ 5 = 2.4 hours
Work completed by each2 × 2.4 = 4.8 units3 × 2.4 = 7.2 units4.8 + 7.2 = 12 units
Portion of job completed4.8 ÷ 12 = 2/57.2 ÷ 12 = 3/5


Therefore, Machine B doesn't have to complete 4.8 units or 2/5 of the job because of Machine A's help.
Substitute x=6, y-=4. This matches our algebraic answer of y/(x+y) = 4/(6+4) = 4/10 = 2/5.
Answer E
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