The price of 1 gallon of unleaded gasoline at the pump dropped to $2.19 on January 1, 2015, and continued to fall at the rate of 4 cents per month for the next 9 months. Express the price of 1 gallon of unleaded gasoline as a function of time for the period starting January 1, 2015. What was the price of 15 gallons of gasoline on April 1, 2015? On September 1, 2015?