A tap can fill a tank in 16 minutes and another can empty it in8 minutes. If the tank is already half full and both the tanks are oped together, the tank will be:

A. filled in 12 min   
B. filled in 8 min
C. emptied in 12 min   
D. emptied in 8 min
Answer:     D

Explanation:   

Rate of waste pipe being more, the tank will be emptied when both the pipes are opened.
Net emptying work done by both in 1 min = (1/8) - (1/16) = 1/16
Now, full tank will be emptied by them in 16 min.
Half full tank will be emptied in 8 min.