Machine A can complete a job in 12 hours, and Machine B can do the

Question

Machine A can complete a job in 12 hours, and Machine B can do the same job at 1/5 of the rate of Machine A. If Machine A completes 3/4 of the job and then Machine B finishes the remaining portion, what is the total amount of time taken by Machine A and Machine B?

(A) 18 hours
(B) 20 hours
(C) 24 hours
(D) 28 hours
(E) 30 hours

(A) 18 hours
(B) 20 hours
(C) 24 hours
(D) 28 hours
(E) 30 hours

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Machine A can do a job in 12 hours at a constant rate and Machine B can do the same job at a 1/5 constant rate of machine A. If machine A does 3/4 of the job and then machine B do the rest of the job, what is the total amount of hours done by machine A and machine B?

VivekSri · Answer

OA is C

Abhishek009 · Answer

Let total work be 60 units...

Efficiency of A is 5 units/hr, Efficiency of B is 1 unit/hour

Work completed by A is 3/4*60 = 45 Units, thus 15 units are left....

Time required by A to complete 45 units is 45/5 = 9 Hours...

Time required by B to complete 15 units is 15/1 = 15 Hours...

SO, Total Time taken is 9 +15 = 24, Answer must be (C)

TheGriffin · Answer

should be C ) use 1/time as rate and its easy then.

ruis · Answer

I used the percent approach presented on the Tips and Hints post (https://gmatclub.com/forum/tips-and-hin ... 72096.html)
We assume that the task is 100%
A does 100% of the job in 12 hours (100/12). So A will take x hours to complete 3/4 (75%) of the job -- 100x = 12*75; x = 900/100; x = 9 hours
B does 100% of the job at 1/5 of the rate of A. This means, machine B es SLOWER than machine A.
Use the reciprocal to understand machines B work rate -- 5/1 of 12 = 60 hours. Hence machine B does 100% of the job in 60 hours (100/60).
How many minutes does machine B take to complete the remaining 25% of the job (100%-75% already completed by machine A)?
100x = 25*60; x = 1500/100; x= 15

Now we add 15+9 = 24

Hope it helps!

bumpbot · Answer

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Machine A can complete a job in 12 hours, and Machine B can do the

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Machine A can complete a job in 12 hours, and Machine B can do the

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