The volume of a sphere is given by the formula V = 4/3 * pi * r^3, where r is the radius of the sphere. Since the diameter of the copper balls is given as 3 inches, the radius is 1.5 inches. Plugging this into the formula, we get V = 4/3 * 3.14 * (1.5)^3 = 14.13 cubic inches per ball.
To find the total volume of copper needed to make 90 balls, we can multiply the volume of one ball by the number of balls: 14.13 cubic inches/ball * 90 balls = 1,271.7 cubic inches of copper.