Worker A takes 10 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?  Choose the correct choice from the following answers
A) 40 days
B) 40 / 9 days
C) 39 / 9 days
D) 6 days
E) 60 / 9 days

Rationale: "a ' s one hour work = 1 / 10 . b ' s one hour work = 1 / 15 . ( a + b ) ' s one hour work = 1 / 10 + 1 / 15 = 5 / 30 = 1 / 6 . both a & b can finish the work in 6 days d"
Answer: D