I do not understand why the rate of change of the area with respect to the radius, evaluated at r=20 does not give us an answer? Surely this would tell you what the rate of change of the area would be when r=20 How do we know that they want the rate of change of area with respect to time, evaluated when r=20?
Is it because of the language used. Because the way the question says it is as if it wants the derivative of the area function evaluated at r=20. It doesn't mention anything about time - why do we make this assumption or am I missing something?
