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
The minimum cost (although not asked) is:
C = 0.25(160)² - 80(160) + 30000
C = $23,600
The answer is 160 units.
