The outlet pipe will take 18 hours to empty the tank.
St<u>ep-by-step explanation:</u>
Number of hours taken by inlet pipe to fill the tank=6
work done by the inlet pipe in 1 hour=1/6
Number of hours taken by the inlet pipe to fill the tank with the outlet pipe open=9
work done by the inlet pipe in 1 hour with outlet pipe open=1/9
we have to determine the time taken by the outlet pipe to empty the tank
The inlet and outlet pipe does the opposite work.
The outlet pipe takes 18 hours to empty the tank.