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?

2hrs
2hrs 20 min
2hrs 40 min
6hrs
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?

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

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

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