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

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