A can do a certain job in 25 days which B alone can do in 20 days. A started the work and was joined by B after 10 days. The number of days taken in completing the work was

A. 12 and half days   
B. 15 days
C. 14 and 2/9 days   
D. 16 and 2/3 days
Answer:     D

Explanation:     

Work done by A in l0 days = (1/25) *10 = 2/5.
Remaining work = 1 - (2/5) = 3/5
(A+B)s 1 days work = (1/25) + (1/20) = 9/100
9/100 work is done by them in 1 day.
hence 3/5 work will be done by them in (100/9) * (3/5) = 20/3days.
Total time taken = (10 + 20/3) = 16 * (2/3) days.