C(n) =75n2 - 1800n + 60 000....if you have had Calculus, this can be determined rather quickly.....but let's assume you haven't......
This function is a parabola that turns "upward"....the quantity that will minimize the cost will be the x coordinate of the vertex (the point at the "bottom" of the curve) found by this "formula"...
x = - b/2a in our function, b = -1800 and a = 75 .....therefore.....
-(-1800)/[2(75)] = 1800/150 = 12 items
Here's the graph.......https://www.desmos.com/calculator/gzsvoswise