1.  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: Option 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 + 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