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 :

A. 1/4
B. 1/10
C. 7/15
D. 8/15
Answer: D

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