Worker A takes 5 hours to do a job. Worker B takes 15 hours to do the same job. How long it take both A & B, working together but independently, to do the same job?  Options:  A) 15 / 4  B) 40 / 9  C) 50 / 9  D) 60 / 9  E) 80 / 9

Rationale: "one day work of a = 1 / 5 one day work of b = 1 / 15 so one day work of a and b together = 1 / 5 + 1 / 15 = 4 / 15 so total days required = 15 / 4 answer : a"
Answer: A