Juan walks up to a tank of water at the aquarium that can hold 20 gallons of water. When it is active, a drain empties, water from the tank at a constant rate. When Juan sees the tank. It contains 8 gallons of water. Five minutes later, the tank contains 6 gallons on water. At what rate is the amount of water in the tank changing? Use a signed number and include the unit of measurement in your answer.