1. A cistern can be filled by two pipes A and B in 4 hours and 6 hours respectively. When full, the tank can be emptied by a third pipe C in 8 hours. If all the taps be turned on at the same time, the cistern will be full in:

2. One tap can fill a cistern in 2 hours and another can empty the cistern in 3 hours. How long will they take to fill the cistern if both the taps are opened?

3. A tank can be filled by one tap in 20 min, and by another in 25 min. both the taps are kept open for 5 min, and then the second is turned off. In how many minutes more is the tank completely filled?