A can do a work in 20days, B in 30days. A alone start the work after 10days he left work. In how many days B can finish the work? Choose the correct option
A) 30 days B) 25 days C) 20 days D) 10 days E) 15 days

Rationale: "10 days work of a = 10 / 20 = 1 / 2 remaining work = 1 - 1 / 2 = 1 / 2 b can finish 1 / 2 work = 30 * 1 / 2 = 15 days answer is e"
Answer: E