The Tank not empty Problem sent in by one of our readers is as shown below 
Question :
Q):A water tank is two-fifth full. pipe A can fill a tank in 10 minutes and pipe B can empty it in 6 minutes. If both the pipes are open,how long will it take to empty or fill the tank completely?
a)6min. to empty
b)6min. to fill
c)9min. to empty
d)9min. to fill
Solution :
For the solution You can read the forum thread – http://aptitudeacademy.co.in/forum/index.php?topic=14.0