Pipe A and B can fill a tank in one hour and two hour respectively while pipe C can empty the filled up tank in one hour and fifteen minute. A and C are turned on together at 9am after two hour, only A is closed and B is turned on. When will the tank be emptied?

A fills at 1 tank/hour, B fills at 0.5 tank/hour, and C empties at 0.8 tank/hour (since 1h15m = 5/4 hours). When A and C work together, net rate = 1 - 0.8 = 0.2 tank/hour. In 2 hours, they fill 0.4 tank. After 2 hours, A stops and B joins C, giving net rate = 0.5 - 0.8 = -0.3 tank/hour. To empty 0.4 tank at 0.3 tank/hour takes 4/3 hours = 1 hour 20 minutes. Starting from 9am + 2 hours + 1h20m = 12:20pm.