$5：x=\dfrac{1}{4}:\dfrac{1}{5}$
$\dfrac{1}{4}x=\dfrac{1}{5}\times 5$
$\dfrac{1}{4}x=1$
$x=4$
$\dfrac{x}{9}=\dfrac{3}{7}$
$7x=3\times 9$
$7x=27$
$x=\dfrac{27}{7}$
$x:\dfrac{3}{4}=\dfrac{1}{2}:\dfrac{2}{5}$
$\dfrac{2}{5}x=\dfrac{3}{4}\times \dfrac{{\displaystyle 1}}{{\displaystyle 2}}$
$\dfrac{2}{5}x=\dfrac{3}{8}$
$x=\dfrac{15}{16}$
