Pipes A and B can fill a tank in 15 hours and 20 hours, respectively. Pipe C is an emptying pipe. A and B are opened together for 4 hours, and then, only A is closed and C is opened. It took 19\(1\over5\) hours more to fill the tank. Pipe C alone can empty the full tank in:
1
42 hours
2
48 hours
3
45 hours
4
40 hours