Worker A takes 5 hours to do a job. Worker B takes 10 hours to do the same job. How long it take both A & B, working together but independently, to do the same job?  Choose the correct choice from the following
A) 20 / 9  B) 40 / 9  C) 50 / 9  D) 60 / 9  E) 10 / 3

Rationale: "one day work of a = 1 / 5 one day work of b = 1 / 10 so one day work of a and b together = 1 / 5 + 1 / 10 = 3 / 10 so total days required = 10 / 3 answer : e"
Answer: E