Olivia walks up to a tank of water that can hold up to 50 gallons. When it is active, a drain empties water form the tank at a constant rate. When Olivia first sees the tank, it contains 38 gallons of water. Four minutes later, the tank contains 18 gallons of water. a.) 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.