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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?
  • a)
    (x – y)/ (x + y)
  • b)
    x / (y – x)
  • c)
    (x + y) / xy
  • d)
    y / (x – y)
  • e)
    y / (x + y)
Correct answer is option 'E'. Can you explain this answer?
Verified Answer
Machine A can complete a certain job in X hours. Machine B can complet...
We can solve this problem as a VIC (Variable In Answer Choice) and plug in values for the two variables, x and y. Let's say x = 2 and y = 3. 
Machine A can complete one job in 2 hours. Thus, the rate of Machine A is 1/2.
Machine B can complete one job in 3 hours. Thus, the rate of Machine B is 1/3.
The combined rate for Machine A and Machine B working together is: 1/2 + 1/3 = 5/6. 
Using the equation (Rate)(Time) = Work, we can plug 5/6 in for the combined rate, plug 1 in for the total work (since they work together to complete 1 job), and calculate the total time as 6/5 hours.
The question asks us what fraction of the job machine B will NOT have to complete because of A's help. In other words we need to know what portion of the job machine A alone completes in that 6/5 hours. 
A's rate is 1/2, and it spends 6/5 hours working. By plugging these into the RT=W formula, we calculate that, A completes (1/2)(6/5) = 3/5 of the job. Thus, machine B is saved from having to complete 3/5 of the job. 
If we plug our values of x = 2 and y = 3 into the answer choices, we see that only answer choice E yields the correct value of 3/5.
 
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Most Upvoted Answer
Machine A can complete a certain job in X hours. Machine B can complet...
The combined rate of A and B is:

1/X + 1/Y

To find the fraction of the job that B will not have to complete because of A's help, we need to determine the portion of the job that A will complete. Let's call this portion "p".

Since A and B are working together to complete the job, the combined rate is equal to the rate of the whole job, which is 1. So we can set up the equation:

1/X + 1/Y = 1

Multiplying both sides by XY, we get:

Y + X = XY

Solving for p, we get:

p = X/(X+Y)

This represents the fraction of the job that A will complete. The fraction of the job that B will not have to complete because of A's help is therefore:

1 - p = Y/(X+Y)

So the answer is (b) Y/(X+Y).
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