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 answers
A) 1 / 4  B) 1 / 10  C) 7 / 15  D) 8 / 15  E) 9 / 15

Rationale: "explanation : a ' s 1 day ' s work = 1 / 15 b ' s 1 day ' s work = 1 / 20 ( a + b ) ' s 1 day ' s work = ( 1 / 15 + 1 / 20 ) = 7 / 60 ( a + b ) ' s 4 day ' s work = 7 / 60 x 4 = 7 / 15 therefore , remaining work = 1 - 7 / 15 = 8 / 15 answer is d"
Answer: D