\(\left\{\begin{matrix}
20.3x=2y\\
y=2x\\
2x+3y-2z=40\end{matrix}\right.
\Leftrightarrow
\left\{\begin{matrix}
x=\frac{y}{30}
\\ y=2z\\
2.\frac{y}{30}+3y-y=40\end{matrix}\right.\)
\(\left\{\begin{matrix}
x=\frac{y}{30}
\\ y=2z\\
\frac{2y}{30}+2y=40\end{matrix}\right.
\Leftrightarrow
\left\{\begin{matrix}
x=\frac{y}{30}
\\ z=\frac{y}{2}\\
y(\frac{2}{30}+2)=40\end{matrix}\right.\)
\(\Leftrightarrow
\left\{\begin{matrix}
x=\frac{20}{31}
\\ z=\frac{300}{31}\\
y=\frac{600}{31}\end{matrix}\right.
\)
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