From calculus, to determine the maxima or minima of the graph, get the derivative of the equation and equate to zero. So, we derive first the equation of the graph and equate to zero.
C = 0.25x² - 80x + 30000 dC/dx = 0 = 0.5x - 80 + 0 0.5x = 80 x = 80/0.5 x = 160 units
