Big engineering calculus problem?

A hemispherical dome of radius 20 feet is to be given 7 coats of paint, each of which is 1/100 inch thick. Use linear approximation to estimate the volume of paint needed for the job. ( Hint: Approximate the change in the volume of a hemisphere when the radius increases from 20 feet to 20 feet 7/100 inches. Don't forget to make sure this increased radius is in terms of feet. ) It will take approximately cubic feet of paint to apply all 7 coats of paint.