Two pipes, when working one at a time can fill a cistern in 2 hours and 3 hours, respectively while a third pipe can drain the cistern empty in 6 hours. All the three pipes were opened together when the cistern was 1/6 full. How long will it take for the cistern to be completely full?
1
1 hour
2
1 hour 20 minutes
3
1 hour 30 minutes
4
1 hour 15 minutes