The yield, Y , of an apple orchard (measured in bushels of apples per acre) is a function of the amount x of fertilizer in pounds used per acre. Suppose

Y = f(x) = 320 + 140x − 10x².

(a) What is the yield if 5 pounds of fertilizer is used per acre?
(b) Find f¨(5). Give units with your answer and interpret it in terms of apples and fertilizer.
(c) Given your answer to part (b), should more or less fertilizer be used? Explain.