Three pipes are attached to a tank. The first one fills it completely in 5 hours, the second one fills it completely in 4 hours and the third one empties it in 20 hours. If all the three pipes are opened simultaneously at 7 a.m., at what time will the tank be completely filled?
1
9:30 a.m.
2
11:00 a.m.
3
10:00 a.m.
4
10:30 a.m.