Machine A can finish a job in 4 hours, Machine В can finish the job in 2 hours, and Machine С can finish the job in 6 hours. How many hours will it take for A, B, and С together to finish the job?
Choose the correct choice from the following answers
A) 4 / 3
B) 6 / 5
C) 8 / 7
D) 10 / 9
E) 12 / 11

Rationale: "the combined rate is 1 / 4 + 1 / 2 + 1 / 6 = 11 / 12 of the job per hour . the time to complete the job is 12 / 11 hours . the answer is e ."
Answer: E