David completes a piece of work in 5 days, John completes the same work in 9 days. If both of them work together, then the number of days required to complete the work is?  Select:
A) 3.2 days B) 8 days C) 10 days D) 12 days E) 14 days

Rationale: ifa can complete a work in x days and b can complete the same work in y days , then , both of them together can complete the work in x y / x + y days . that is , the required no . of days = 5 × 9 / 14 = 3.2 days answer : a
Answer: A