28 / 5x(5-x) = 300
5x(5-x) = 28/300
25x - 5x^2 = 7/75
1875x - 375x^2 = 7
375x^2 - 1875x + 7 = 0
x = {-(-1875) + [(-1875)^2 - 4(375)(7)]^(1/2)} / 2(375) 或 {-(-1875) - [(-1875)^2 - 4(375)(7)]^(1/2)} / 2(375)
= [1875 + 3505125^(1/2)] / 750 或 [1875 - 3505125^(1/2)] / 750
= 5.00 或 0.00374