$$\frac{ z4 }{ 5x } =y$$
$\left\{\begin{matrix}x=\frac{z_{4}}{5y}\text{, }&z_{4}\neq 0\text{ and }y\neq 0\\x\neq 0\text{, }&y=0\text{ and }z_{4}=0\end{matrix}\right.$
$y=\frac{z_{4}}{5x}$
$x\neq 0$
