A can do a work in 15 days and B in 20 days. If they work on it together for 3 days, then the fraction of the work that is left is Select:
A) 8 / 17  B) 7 / 15  C) 3 / 15  D) 8 / 15  E) 13 / 20

Rationale: "person ( a ) ( b ) ( a + b ) time - ( 15 ) ( 20 ) ( - ) rate - ( 20 ) ( 15 ) ( 35 ) work - ( 300 ) ( 300 ) ( 300 ) therefore a + b requires ( 300 / 35 ) days to complete entire work for 1 st 4 days they work 35 * 3 = 105 remaining work is 300 - 105 = 195 remaining fraction of work is = 195 / 300 = 13 / 20 answer e"
Answer: E