The cost (in dollars) of producing $x$ units of a certain commodity is $$C(x) = 2500 + 14 x +0.2 x^2.$$

(a) Find the average rate of change of $C$ with respect to $x$ when the production level is changed

(i) from $x = 100$ to $x = 105$.
Average rate of change =

(ii) from $x = 100$ to $x = 101$.
Average rate of change =

(b) Find the instantaneous rate of change of $C$ with respect to $x$ when $x = 100$. (This is called the marginal cost.)
Instantaneous rate of change =