Worker A takes 10 hours to do a job. Worker B takes 12 hours to do the same job. How long it take both A & B, working together but independently, to do the same job? Pick one
A) 40 days B) 40 / 9 days C) 60 / 11 days D) 30 / 9 days E) 60 / 9 days

Rationale: "a ' s one hour work = 1 / 10 . b ' s one hour work = 1 / 12 . ( a + b ) ' s one hour work = 1 / 10 + 1 / 12 = 11 / 60 . both a & b can finish the work in 60 / 11 days c"
Answer: C