As a snowball melts, its surface area and volume decrease. The surface area, S, in square centimetres, is modelled by the equation S = 4π r2, where r is the radius, in centimetres. The volume, V, in cubic centimetres, is modelled by the equation V = (4/3)π r3.

a) How do you determine the average rate of change of the surface area and of the volume as the radius decreases from 25 cm to 20 cm?

b) How do you determine the instantaneous rate of change of the surface area and the volume when the radius is 10 cm?