G1.3 If $x, y$ and $z$ are real numbers, $x y z \neq 0,2 x y=3 y z=5 x z$ and $c=\frac{x+3 y-3 z}{x+3 y-6 z}$, find the value of $c$.
$\begin{array}{l}\frac{2 x y}{x y z}=\frac{3 y z}{x y z}=\frac{5 x z}{x y z} \Rightarrow \frac{2}{z}=\frac{3}{x}=\frac{5}{y} \Rightarrow x: y: z=3: 5: 2 \\ \text { Let } x=3 k, y=5 k, z=2 k \\ c=\frac{x+3 y-3 z}{x+3 y-6 z}=\frac{3 k+15 k-6 k}{3 k+15 k-12 k}=2\end{array}$