A can do a job in 15days and B in 20days. 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 choices.
A) 2/15 B) 8/15 C) 3/11 D) 1/12 E) 6/13
A's 1 day work = 1/15   8/15's 1day work = 1/20   A+8/15 1day work = 1/15 + 1/20 = 7/60   A+8/15 4days work = 7/60*4 = 7/15   Remaining work = 1 - 7/15 = 8/15   Answer is 8/15
<result>B</result>
A can do a job in 15days and B in 20days. If they work on it together for 4 days, then the fraction of the work that is left is?