\[b_{3} = 1;\ \ \ \ \ b_{5} = \frac{1}{4}:\]
\[\left\{ \begin{matrix} b_{1} \cdot q^{2} = 1 \\ b_{1} \cdot q^{4} = \frac{1}{4} \\ \end{matrix}\text{\ \ \ \ \ } \right.\ \]
\[\frac{1}{q^{2}} = 4\ \ \ \ \ \]
\[q^{2} = \frac{1}{4}\text{\ \ \ \ \ \ }\]
\[q = \pm \frac{1}{2}\]
\[Ответ:\ \ q = \pm \frac{1}{2}.\]