[tex]\it \dfrac{^{2)}x}{\ \ 3}=\dfrac{y}{6} \Rightarrow \dfrac{2x}{6}=\dfrac{y}{6} \Rightarrow y=2x\ \ \ \ \ (1)\\ \\ \\ \dfrac{y}{2}=z\ \stackrel{(1)}{\Longrightarrow}\ \dfrac{2x}{2}=z \Rightarrow z=x\ \ \ \ \ (2)\\ \\ \\ x+2y+z=18\ \stackrel{(1),(2)}{\Longrightarrow}\ \ x+4x+x=18 \Rightarrow 6x=18 \Rightarrow x=3\ \ \ \ \ \ (3)[/tex]
[tex]\it (1),\ (3) \Rightarrow y=2\cdot3=6\\ \\ (2),\ (3) \Rightarrow z=x=3[/tex]
