Đáp án:
$\begin{array}{l}
a)4x = 3y\\
\Leftrightarrow \dfrac{x}{3} = \dfrac{y}{4} = \dfrac{{x + y}}{{3 + 4}} = \dfrac{{28}}{7} = 4\\
\Leftrightarrow \left\{ \begin{array}{l}
x = 4.3 = 12\\
y = 4.4 = 16
\end{array} \right.\\
Vậy\,x = 12;y = 16\\
b)\dfrac{x}{3} = \dfrac{y}{4} = \dfrac{{2x}}{6} = \dfrac{{5y}}{{20}} = \dfrac{{2x + 5y}}{{6 + 20}} = \dfrac{{10}}{{26}} = \dfrac{5}{{13}}\\
\Leftrightarrow \left\{ \begin{array}{l}
x = \dfrac{5}{{13}}.3 = \dfrac{{15}}{{13}}\\
y = \dfrac{5}{{13}}.4 = \dfrac{{20}}{{13}}
\end{array} \right.\\
Vậy\,x = \dfrac{{15}}{{13}};y = \dfrac{{20}}{{13}}\\
c)\left\{ \begin{array}{l}
\dfrac{x}{2} = \dfrac{y}{3} \Leftrightarrow \dfrac{x}{8} = \dfrac{y}{{12}}\\
\dfrac{y}{4} = \dfrac{z}{5} \Leftrightarrow \dfrac{y}{{12}} = \dfrac{z}{{15}}
\end{array} \right.\\
\Leftrightarrow \dfrac{x}{8} = \dfrac{y}{{12}} = \dfrac{z}{{15}} = \dfrac{{x + y - z}}{{8 + 12 - 15}} = \dfrac{{10}}{5} = 2\\
\Leftrightarrow \left\{ \begin{array}{l}
x = 2.8 = 16\\
y = 2.12 = 24\\
z = 2.15 = 30
\end{array} \right.\\
Vậy\,x = 16;y = 24;z = 30
\end{array}$
