A can do a job in 15days and B in 20days. If they work on it together for 3 days, then the fraction of the work that is left is?	Choose the correct choice:
A) 2 / 15
B) 8 / 15
C) 13 / 20
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 3 days work = 7 / 60 * 3 = 7 / 20 remaining work = 1 - 7 / 20 = 13 / 20 answer is c
Answer: C