Pipe A can fill an empty tank in 5 hours while pipe B can empty the full tank in 6 hours. If both are opened at the same time in the empty tank, how much time will they take to fill it up completely?
Number of hours Pipe A requires to fill an empty tank : 5 hours
Number of hours Pipe B requires to empty the full tank : 6 hours
Amount of water filled by Pipe A in empty tank in one hour: 
Amount of water Pipe B empties in one hour: 
Amount of water filled by Pipe A and Pipe B together in one hour: 
= 
= 
They can fill the tank together in 30 hours.
15