4. A company determines that the cost in dollars of producing x units is given by the following cost function: C(x) = 400x-16x2,0 < x < 25. [5 marks] a) What is the largest number of units that must be made for the cost to be 2400 dollars? b) Find the marginal cost. c) If the cost is 2400 dollars, and the marginal cost is negative, how many units are made? d) Find the largest interval where the marginal cost is negative. e) Find the largest interval where the graph of C(x) is concave down.