A chemical company needs to paint forty chemical tanks including the top and the bottom of the tanks. Each tank has a height of 7 ft and a diameter of 4 ft. Paint can be purchased in 1-gallon cans or in 5-gallon cans that cost the same as 4 1-gallon cans. Each gallon of paint will cover 350 square feet. How many of each should be purchased to keep the cost of painting the tanks as low as possible?