Đáp án:
$\begin{array}{l}
\dfrac{{12x - 15y}}{7} = \dfrac{{20z - 12x}}{9} = \dfrac{{15y - 20z}}{{11}}\\
= \dfrac{{12x - 15y + 20z - 12x + 15y - 20z}}{{7 + 9 + 11}}\\
= \dfrac{0}{{27}} = 0\\
\Rightarrow \left\{ \begin{array}{l}
12x = 15y\\
20z = 12x\\
15y = 20z
\end{array} \right.\\
\Rightarrow 12x = 15y = 20z\\
\Rightarrow \dfrac{{12x}}{{60}} = \dfrac{{15y}}{{60}} = \dfrac{{20z}}{{60}}\\
\Rightarrow \dfrac{x}{5} = \dfrac{y}{4} = \dfrac{z}{3} = k\\
\Rightarrow \left\{ \begin{array}{l}
x = 5k\\
y = 4k\\
z = 3k
\end{array} \right.\left( {k \in Z} \right)
\end{array}$
