\begin{array}{l} a. \\ 12 - 4x \le 36 + 2x \\ 12 - 6x \le 36 \\ - 6x \le 24 \\ x \ge - 4 \\ \\ b. \\ \eqalign{\frac{{6 - 2x}}{5} \le \frac{{4 - x}}{4}} \\ 4\left( {6 - 2x} \right) \le 5\left( {4 - x} \right) \\ 24 - 8x \le 20 - 5x \\ - 3x \le - 4 \\ x \ge 1\frac{1}{3} \\ \\ c. \\ 200 - (80 - x) > 4(x + 15) \\ 200 - 80 + x > 4x + 60 \\ 120 + x > 4x + 60 \\ - 3x > - 60 \\ x < 20 \\ \\ d. \\ 60 - x^2 \ge 4x \\ - x^2 - 4x + 60 \ge 0 \\ {\rm{Gelijkheid:}} \\ - x^2 - 4x + 60 = 0 \\ x^2 + 4x - 60 = 0 \\ (x + 10)(x - 6) = 0 \\ x = - 10 \vee x = 6 \\ {\rm{Oplossing:}} \\ - 10 \le x \le 6 \\ \\ e. \\ x^3 < 80x - 2x^2 \\ x^3 + 2x^2 - 80x < 0 \\ {\rm{Gelijkheid:}} \\ x^3 + 2x^2 - 80x = 0 \\ x(x^2 + 2x - 80) = 0 \\ x(x + 10)(x - 8) = 0 \\ x = 0 \vee x = - 10 \vee x = 8 \\ {\rm{Oplossing:}} \\ x<-10\vee 0<x<8\\ \end{array}