If 1 gallon of paint covers 400 square feet about how many gallons of paint are needed to paint the outside of a spherical tank of diameter 20 feet?

Surface area of a sphere = 4*pi*radius2

Area = 4*pi*102 = 400*pi square feet

Amount needed in gallons = (400*pi)/400 = pi (3.141592654)

Just over 3 gallons will be needed.