How many gallons of paint are necessary to paint the walls of a 15 by 12 room with an 8 ceiling if one gallon of paint covers 80 square feet?

To calculate the amount of paint needed, first find the total wall area of the room. The room has two walls that are 15 feet long and two walls that are 12 feet long, with a height of 8 feet. The total wall area is (2 \times (15 + 12) \times 8 = 432) square feet. Given that one gallon covers 80 square feet, you would need (432 \div 80 = 5.4) gallons of paint, so you should purchase 6 gallons to ensure complete coverage.