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13. Let f(t) be the amount of oxygen (in suitable units) in a lake t days after sewage is dumped into the lake, and suppose that f(t) is given approximately by the following. At what time is the oxygen content increasing the fastest? 15 225 f(t) = 1 - - + t+ 15 (t+ 15)- Therefore, the oxygen content of the lake is increasing the fastest after days. 14. Given the cost function C(x) = 3x - 2x + 2x + 7, find the minimum marginal cost. The minimum marginal cost is $ (Do not round until the final answer. Then round to two decimal places as needed.) 15. An artist is planning to sell signed prints of her latest work. If 50 prints are offered for sale, she can charge $600 each. However, if she makes more than 50 prints, she must lower the price by $5 for each print in excess of the 50. How many prints should the artist make to maximize her revenue? Let x be the number of prints and p the price per print. The demand equation is p = and the revenue function is R(X) = The artist should make prints. (Round to the nearest whole number as needed.)