The graph above shows the height, in feet, of water in a tank over a particular ten minute period. At the beginning of the ten minutes, a hose at the top of the tank was turned on. Four minutes later, the hose was turned off. Three minutes after that, a drain at the bottom of the tank was opened.

On a different day, the same tank is filled with water to a height of four feet. Then, a student opens the drain, turns on the hose, and begins a timer simultaneously. How much time elapses before the tank is empty?