A can do a job in 15days and B in 20days. If they work on it together for 4 days, then the fraction of the work that is left is?
Answers
A) 2 / 15
B) 8 / 15
C) 3 / 11
D) 1 / 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 4 days work = 7 / 60 * 4 = 7 / 15 remaining work = 1 - 7 / 15 = 8 / 15 answer is b"
Answer: B