A painter is going to paint a room shaped like a cube, with square walls that are 20 ft. on each side. A gallon of paint can be used to cover 400 square feet. How many gallons of paint should the painter buy if he is going to paint the four walls of the room?
5 gallons
1 gallon
4 gallons
6 gallons
The total surface area of the four walls is 4 * (20 ft * 20 ft) = 1600 square feet.
To cover this area, the painter needs 1600 square feet / 400 square feet per gallon = <<1600/400=4>>4 gallons of paint. Answer: \boxed{4}.