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) 8
B) 8 / 11
C) 8 / 13
D) 8 / 15
E) none

Rationale: a ' s 1 day ' s work = 1 ; 15 b ' s 1 day ' s work = 1 ; 20 ( a + b ) ' s 1 day ' s work = ( 1 + 1 ) = 7 . 15 20 60 ( a + b ) ' s 4 day ' s work = ( 7 x 4 ) = 7 . 60 15 therefore , remaining work = ( 1 - 7 ) = 8 . 15 15 option d
Answer: D