A and B can together finish a work in 30 days. They worked at it for 20 days and then B left. The remaining work was done by A alone in 20 more days. A alone can finish the work in:

A. 48 days   
B. 54 days
C. 50 days   
D. 60 days
Answer:     D

Explanation:     

Work done by A and B in 20 days = (1/30 * 20) = 2/3
Remaining work= ( 1 -2/3) = 1/3 Now,1/3 work is done by A in 20days
Whole work will be done by A in (20x3) = 60 days.