Đáp án:
$\begin{array}{l}
c)3x = 2y \Leftrightarrow \left\{ {\dfrac{x}{2} = \dfrac{y}{3}} \right. \Leftrightarrow \dfrac{x}{{10}} = \dfrac{y}{{15}}\\
7y = 5z \Leftrightarrow \dfrac{y}{5} = \dfrac{z}{7} \Leftrightarrow \dfrac{y}{{15}} = \dfrac{z}{{21}}\\
\Leftrightarrow \dfrac{x}{{10}} = \dfrac{y}{{15}} = \dfrac{z}{{21}} = \dfrac{{x - y + z}}{{10 - 15 + 21}} = \dfrac{{32}}{{16}} = 2\\
\Leftrightarrow \left\{ \begin{array}{l}
x = 20\\
y = 30\\
z = 42
\end{array} \right.\\
Vậy\,x = 20;y = 30;z = 42\\
d)\dfrac{{2x}}{3} = \dfrac{{3y}}{4} = \dfrac{{4z}}{5}\\
\Leftrightarrow \dfrac{{2x}}{{3.12}} = \dfrac{{3y}}{{4.12}} = \dfrac{{4z}}{{5.12}}\\
= \dfrac{x}{{18}} = \dfrac{y}{{16}} = \dfrac{z}{{15}} = \dfrac{{x + y + z}}{{18 + 16 + 15}} = \dfrac{{49}}{{49}} = 1\\
\Leftrightarrow \left\{ \begin{array}{l}
x = 18\\
y = 16\\
z = 15
\end{array} \right.\\
Vậy\,x = 18;y = 16;z = 15
\end{array}$
