According to the measurements of the building, it has two sides that have an area of 28x 26 = 728 square feet and two sides that have an area of 42 x 26 = 1092 square feet. Which means the total area of the building is (1092 + 728 ) x 2 = 3640 square feet. If 480 square feet of this area is windows, then the area to be painted is a total of 3160 square feet. If one gallon of paint covers 480 square feet, then the total amount of paint that is needed is 3160 / 400 = 7.9 gallons.