\[\left( \frac{x}{4} \right)^{2}\ :\left( \frac{x}{2} \right)^{3} = \frac{x^{2}}{16}\ :\frac{x^{3}}{8} =\]
\[= \frac{x^{2} \cdot 8}{16 \cdot x^{3}} = \frac{1}{2x}\]