While working alone at their constant rates computer X can process 240

Question

While working alone at their constant rates computer X can process 240 files in 4 hours, and comp Y 240 files in 8 hrs. If all files processed by these comp. are the same size how long would it take the two comp. working at the same time at their respective constant rates , to process a total of 240 files?

A. 2hrs
B. 2hrs 20 min
C. 2hrs 40 min
D. 6hrs
E. 6hrs 20 min

A. 2hrs
B. 2hrs 20 min
C. 2hrs 40 min
D. 6hrs
E. 6hrs 20 min

i did as under
X in one hour = 240/4 = 60 files
Y in one hour = 240/8 = 30 fles
X+ Y in one hour 60+30=90 files
So together they will take (240/90)= 2 hours 40 minutes. 
So C is the answer.
I do not know the OA.
Am I right?

subhashghosh · Accepted Answer

Yes, your answer is correct.

Btw, 240 files information is irrelevant here, as both do the same work, it can be regarded as 1 unit of work.

In 1 hr, both finish = 1/8 + 1/4 = (1+2)/8 part of work

So total in 8/3 hrs

= 2 2/3 hrs

= 2 hrs 40 min

shrouded1 · Answer

Computer 1 processes 60 files an hour
Computer 2 processes 30 files an hour

Combined they will process 90 files an hour

For 240 files, they will take (240/9) hours ... which is 2h40m

fluke · Answer

Yes, you are right.

You have solved it by unitary method;

90 files -> 1 hour
1 file -> 1/90 hour
240 files -> (1/90)* 240 hours

gmatprep2011 · Answer

X - 60 files per hour
Y - 30 files per hour
X & Y together - 90 files per hour
90+90=180
We need 60 more
So 60/90 of an hour i.e. 2/3 of an hour = 40 min
So 2 hours and 40 min...

MBAhereIcome · Answer

X does 240 in 4hrs = 60 in 1 hr
Y does 240 in 8hrs = 30 in 1hr

X+Y = 90 in 1hr
240 in = 240/90 = 2.6666 hrs = 2hrs 40mins

aks1985 · Answer

I like this approach. But can someone tell me why 3/8 was flipped to 8/3 to get the final answer?

Spidy001 · Answer

X 60 files/hour
Y 30 files/hour

together , they will take 240/(60+30) hours to process 240 files

= 240/90 = 2hr 40 min

Spidy001 · Answer

Here we are considering work as 1 unit of work.

X --- does 1 unit of work in 4 hours => rate of x = 1/4
Y ---- does 1 unit of work in 8 hours =>rate of y = 1/8

together 
 their rate is 1/4+1/8 = 3/8
 work - same 1 unit of work = 1
 time it will take do the unit work = 1/(3/8) = 8/3 hours

Hope its clear now.

amit2k9 · Answer

X in 1 hr = 60 and y in 1 hr = 30 files.
in 2 hrs X + Y = 120 + 60 = 180
remaining = 60

hence in 40 mins X + Y = 40 + 20 = 60

thus 2:40 C is the cue.

ScottTargetTestPrep · Answer

The rate of computer X is 240/4 = 60 files in 1 hour and the rate of computer Y is 240/8 = 30 files in 1 hour, so the combined rate is 90 files in 1 hour. Thus, it takes 240/90 = 2 60/90 hours = 2 ⅔ hours = 2 hours and 40 minutes to complete 240 files when both computers work together.

Answer: C

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While working alone at their constant rates computer X can process 240

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While working alone at their constant rates computer X can process 240

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