A type R machine can complete a job in 5 hours and a type B machine can complete the job in 7 hours. How many hours will it take 2 type R machines and 3 type B machines working together and independently to complete the job?
Pick.
A) 1 / 5  B) 29 / 35  C) 5 / 6  D) 35 / 29  E) 35 / 12

Rationale: now d should be the answer . r need 5 hours to complete and b needs 7 hours to compete so 2 r + 3 b will complete 2 / 5 + 3 / 7 or 29 / 35 portion of the job in 1 hour so the whole job will take 35 / 29 hours . . . . = d
Answer: D