A farmer measured the quantity of water in a tank 10 days after it was filled and found it to be 2000 gallons. After the next 5 days, it was measured again
and found to contain 1300 gallons. Assume that the amount of water in the
tank decreases at a constant rate. Find the slope of the linear relation
between L, the number of liters of water consumed, and t, the number of
days after the tank was filled.