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work in 1 hr

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work in 1 hr

by grandh01 » Mon Aug 13, 2012 7:31 pm
Three machines, individually, can do a
certain job in 4, 5, and 6 hours,
respectively. What is the greatest part
of the job that can be done in one hour
by two of the machines working
together at their respective rates?

(A)11/30
(B)9/20
(C)3/5
(D)11/15
(E)5/6
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by coolhabhi » Mon Aug 13, 2012 8:28 pm
Since the greatest part of the work needs to be completed choose those two machines which will take less amount of time.
Those machines would be A, B which take 4 and 5 hours respectively.

Now if they work together for one hour, then they will complete {(1/4) + (1/5)} part of work. That is 9/20.
Answer : B
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Re: work in 1 hr

by Brent@GMATPrepNow » Thu Jun 10, 2021 5:09 am
grandh01 wrote: ↑
Mon Aug 13, 2012 7:31 pm
 Three machines, individually, can do a
certain job in 4, 5, and 6 hours,
respectively. What is the greatest part
of the job that can be done in one hour
by two of the machines working
together at their respective rates?

(A)11/30
(B)9/20
(C)3/5
(D)11/15
(E)5/6
Assign a "nice" value to the job (i.e., a value that works well with the given numbers, 4, 5 and 6)
Since 4, 5 and 6 all divide into 60, let's say that completing the job means making 60 widgets

So, the first machine can make 60 in 4 hours, which means its rate is 15 widgets per hour.
The second machine can make 60 in 5 hours, which means its rate is 12 widgets per hour.
The 3rd machine can make 60 in 6 hours, which means its rate is 10 widgets per hour.

So, the first machine and second machine are the two fastest.
Their combined rate = 15 + 12 = 27 widgets per hour
So, in one hour, those two machines can make 27 widgets, which means they can complete 27/60 of the job.

27/60 simplifies to be 9/20
Answer: B
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