A can do a job in 15days and B in 30days. If they work on it together for 4 days, then the fraction of the work that is left is?
Choose the correct choice from the following answers: A) 2 / 15 B) 3 / 5 C) 3 / 11 D) 1 / 12 E) 6 / 13

Rationale: "a ' s 1 day work = 1 / 15 b ' s 1 day work = 1 / 30 a + b 1 day work = 1 / 15 + 1 / 30 = 3 / 30 = 1 / 10 a + b 4 days work = 1 / 10 * 4 = 2 / 5 remaining work = 1 - 2 / 5 = 3 / 5 answer is b"
Answer: B