A can do a job in 10days and B in 20days. If they work on it together for 4 days, then the fraction of the work that is left is?
Pick one:
A) 2 / 5
B) 8 / 15
C) 3 / 11
D) 1 / 12
E) 6 / 13

Rationale: "a ' s 1 day work = 1 / 10 b ' s 1 day work = 1 / 20 a + b 1 day work = 1 / 10 + 1 / 20 = 3 / 20 a + b 4 days work = 3 / 20 * 4 = 3 / 5 remaining work = 1 - 3 / 5 = 2 / 5 answer is a"
Answer: A