2 gallons of paint are needed.
Granted there are no doors or windows, a 8ft by 100ft wall will have an area of 800 square feet (multiply length * height, so 8 * 100 = 800).
If each gallon of paint covers 400 square feet, we need to divide our area by 400. 800/400 = 2.
So, 2 gallons of paint are needed to cover the wall.
