<image>As shown in the figure, a container equipped with an inlet pipe and an outlet pipe starts to fill with water only for 4 minutes, followed by 8 minutes during which water both enters and exits the container, and then the inlet pipe is closed until the water inside the container is completely drained. Assuming the water inflow rate and outflow rate per minute are two constants, and given the partial relationship between the water volume y (in liters) in the container and time x (in minutes), how many minutes does it take for the container to completely empty from the moment the inlet pipe is closed?
