Suppose the profit (in cents) from selling x lb of potatoes is given by
P(x) = 15x + 25x².
Find the average rate of change in profit from selling each of the following amounts.
(a) 6 lb to 7 lb 

(b) 6 lb to 6.5 lb 

(c) 6 lb to 6.1 lb

Find the marginal profit (that is, the instantaneous rate of change of the profit) from selling the following amounts.
(d) 6 lb 

(e) 20 lb 

(f) 30 lb
(g) What is the domain of x?
(h) Is it possible for the marginal profit to be negative here? What does this mean?
(i) Find the average profit function. (Recall that average profit is given by total profit divided by the number produced, or P(x) = P(x)/x.)
(j) Find the marginal average profit function (that is, the function giving the instantaneous rate of change of the average profit function).
(k) Is it possible for the marginal average profit to vary here? What does this mean?