A mail handler needs 3 hours to sort the mail. His assistant takes 6 hours to sort the mail. How many hours will it take for both of them working together to sort the mail?
Answers.
A) 3  B) 2  C) 1  D) 2 1 / 2  E) 1 1 / 5

Rationale: work hrs = ab / ( a + b ) = 18 / 9 = 2 answer is b
Answer: B