Suppose the total cost of manufacturing q units of certain commodity is given by

C (q) = q^3-30q^2+400q+500

a. Computer the cost of manufacturing 20 units.

I understand what to do here. Just plug in 20.

b. Compute the cost of manufacturing the 20th unit.

Isn't it c(20) -c (0) / 20 - 0 ???