A can do a job in 15days and B in 20days. If they work on it together for 5 days, then the fraction of the work that is left is?  Pick one
A) 2 / 15 B) 8 / 15 C) 3 / 11 D) 5 / 12 E) 6 / 13

Rationale: "a ' s 1 day work = 1 / 15 b ' s 1 day work = 1 / 20 a + b 1 day work = 1 / 15 + 1 / 20 = 7 / 60 a + b 5 days work = 7 / 60 * 5 = 7 / 12 remaining work = 1 - 7 / 12 = 5 / 12 answer is d"
Answer: D