$x=30-z-y\text{, }y=-\frac{\sqrt{\left(30z-z^{2}\right)^{2}-3640z}}{2z}-\frac{z}{2}+15\text{, }z\in \mathrm{R}$
$x=30-z-y\text{, }y=\frac{\sqrt{\left(30z-z^{2}\right)^{2}-3640z}}{2z}-\frac{z}{2}+15\text{, }z\in \mathrm{R}\text{, }z\neq 0\text{ and }\left(-zz+30z\right)^{2}-3640z\geq 0$