Worker A takes 7 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?  Select.
A) 70 / 17	B) 40 / 9	C) 50 / 9	D) 60 / 9	E) 80 / 9

Rationale: "one day work of a = 1 / 7 one day work of b = 1 / 10 so one day work of a and b together = 1 / 7 + 1 / 10 = 17 / 70 so total days required = 70 / 17 answer : a"
Answer: A