Jada walks up to a tank of water that can hold up to 20 gallons. When it is active, a drain empties water from the tank at a constant rate. When Jada first sees the tank, it contains 12 gallons of water. Four minutes later, the tank contains 9 gallons of water. At what rate is the amount of water in the tank changing?
How many more minutes will it take for the tank to drain completely?
How many minutes before Jada arrived was the water tank completely full?