While working alone at their constant rates, computer X can process 240 files in 12 hours, and computer Y can process 240 files in 4 hours. If all files processed by these computers are the same size, how many hours would it take the two computers, working at the same time at their respective constant rates, to process a total of 240 files?  Answers:  A) 2  B) 2.4  C) 2.7  D) 3  E) 3.5

Rationale: "both computers together process files at a rate of 240 / 12 + 240 / 4 = 20 + 60 = 80 files per hour . the time required to process 240 files is 240 / 80 = 3 hours the answer is d ."
Answer: D