How many gallons of paint would you need?

Question

You are preparing to paint a grain silo. The silo is a 20-foot tall cylinder with a circumference of 147 feet. One gallon of paint covers 300 square feet. How much paint should you buy to cover all but the bottom of the silo with one coat of paint?

Answer 1

The surface area of the silo's side is 20x147 = 2940 square feet

Then add the area of the top, if flat, is pi x radius squared = pi(r)²

The circumference is pi x the diameter or 2pi(r) = 147

r= 147/2pi

top area = pi(r)² = pi(147/2pi)² = 147²/4pi = 3439.18

total area = sides + top = 2940 + 3439.18 = 6379.18 square feet

divide that by 300 to get the number of gallons needed

6379.18/300 = 21.2639 gallons

a little more than 21 1/4 gallons