A can do a piece of work in 15 days. A does the work for 5 days only and leaves the job. B does the remaining work in 10 days.In how many days B alone can do the work?  Choose one: A) 5 days B) 15 days C) 12 days D) 9 days E) 10 days

Rationale: "explanation : a ’ s 5 day work = 5 * 1 / 15 = 1 / 3 remaining work = 1 - 1 / 3 = 2 / 3 b completes 2 / 3 work in 6 days b alone can do in x days 2 / 3 * x = 10 x = 15 days answer : option b"
Answer: B