Patty paints pink houses for a living. Patty's demand for pink paint is 5 gallons per week. It costs her $15 each time she drives to the paint store to buy some paint. Holding costs are $0.75/gallon/week. The paint store charges $25/gallon if Patty buys less than 20 gallons of paint but only $22/gallon if she buys 20 or more gallons at once. How much pink paint should Patty purchase each time she goes to the paint store? Be sure to show your total cost calculations.