A can do a work in 15 days and B in 20 days. 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 options.
A) 1 / 4 B) 1 / 10 C) 7 / 15 D) 8 / 15 E) 9 / 13

Rationale: "explanation : a ’ s one day work = 1 / 15 b ’ s one day work = 1 / 20 ( a + b ) ’ s one day work is = 1 / 15 + 1 / 20 = 7 / 60 their 4 days work = 4 x 7 / 60 = 7 / 15 remaining work = 8 / 15 answer : option d"
Answer: D