Đáp án:
$\begin{array}{l}
\left\{ \begin{array}{l}
3x = y\\
5y = 4z
\end{array} \right.\\
\Rightarrow \left\{ \begin{array}{l}
\frac{x}{1} = \frac{y}{3} \Rightarrow \frac{x}{4} = \frac{y}{{12}}\\
\frac{y}{4} = \frac{z}{5} \Rightarrow \frac{y}{{12}} = \frac{z}{{15}}
\end{array} \right.\\
\Rightarrow \frac{x}{4} = \frac{y}{{12}} = \frac{z}{{15}} = \frac{{6x}}{{24}} = \frac{{7y}}{{12.7}} = \frac{{8z}}{{15.8}} = \frac{{6x + 7y + 8z}}{{24 + 84 + 120}} = \frac{{456}}{{228}} = 2\\
\Rightarrow \left\{ \begin{array}{l}
x = 8\\
y = 24\\
z = 30
\end{array} \right.
\end{array}$
