You need to find the surface area and the volume. The formula for surface area is 4*3.14(pi)*radius^2 which is equal to 6.5144 inches^2 in this case. The amount of rubber needed to fill it is equal to the volume. The formula for volume of a sphere is 4/3*3.14(pi)*radius^3, just plug in the number (remember radius is half of diameter) and that will give you the answer!
