Given the revenue and cost functions R = 50x - 0.4x² and C = 5x + 15 (in dollars), where x is the daily production (and sales), find the following when 40 units are produced daily and the rate of change of production is 10 units per day.
(a) The rate of change of revenue with respect to time
(b) The rate of change of cost with respect to time
(c) The rate of change of profit with respect to time