Find the surface area of the room by measuring all the areas of the 6 surfaces. This is simple, as each side is repeated, therefore you only need to do 3 equations. But since you don't need to measure the floor, you only need to do the first one once.
15*20 = 300
20*8*2 = 320
8*15*2 = 240
Add all these up to get 860, then multiply it by 2 to get your second coat of paint, which will equal 1,720.
Now you divide that by 300, getting 5.73 gallons of paint required to paint the ceiling and walls.
