Đáp án:
$\begin{array}{l}
a)\dfrac{{x - y}}{{x + 2y}} = \dfrac{3}{4}\\
\Leftrightarrow 4.\left( {x - y} \right) = 3.\left( {x + 2y} \right)\\
\Leftrightarrow 4x - 4y = 3x + 6y\\
\Leftrightarrow x = 10y\\
\Leftrightarrow \dfrac{x}{y} = 10\\
b)\dfrac{x}{y} = \dfrac{2}{3} \Leftrightarrow y = \dfrac{3}{2}.x\\
\dfrac{t}{y} = \dfrac{4}{9} \Leftrightarrow t = \dfrac{4}{9}.y\\
\dfrac{z}{t} = \dfrac{5}{8} \Leftrightarrow z = \dfrac{5}{8}.t\\
\Leftrightarrow z = \dfrac{5}{8}.\left( {\dfrac{4}{9}.y} \right)\\
= \dfrac{5}{{18}}.y\\
= \dfrac{5}{{18}}.\dfrac{3}{2}.x\\
= \dfrac{5}{{12}}.x\\
\Leftrightarrow z = \dfrac{5}{{12}}.x\\
\Leftrightarrow \dfrac{x}{z} = \dfrac{{12}}{5}
\end{array}$
