The inlet pipe can fill a tank in 4 hours while the outlet can empty it in 5 hrs. Both the pipes were opened at 9AM, but after some times the outlet pipe closed and it is observed that the tank was full at 5 PM. At what time was the outlet closed?

Inlet fills in 4 hours, so rate = 1/4 per hour. Outlet empties in 5 hours, so rate = -1/5 per hour. When both open, net rate = 1/4 - 1/5 = 1/20 per hour. When only inlet works, rate = 1/4 per hour. From 9 AM to 5 PM = 8 hours. If outlet closed after x hours, then x/20 + (8-x)/4 = 1. Solving: x/20 + 2 - x/4 = 1, which gives -4x/20 + 2 = 1, so -x/5 = -1, and x = 5 hours. Outlet closed at 9 AM + 5 hours = 2 PM.