As can be seen from the figure, only 4 minutes after opening the water inlet pipe, 20 liters of water will enter, so the speed of the water inlet pipe is 20/4=5 (liter/minute).

When the inlet and outlet pipes are opened at the same time 12-4=8 minutes, and the water inflow is 30-20= 10 liter, the rotation speed of the outlet pipe is 5- 10/8=3.75 liter.

After the water inlet pipe is closed, there is 30 liters of water, and the water discharge time is 30/3.75=8 minutes.

After closing the water inlet pipe, after __8__ minutes, the water in the container is just discharged.