\[b_{3} = 12;\ \ b_{5} = 48\]
\[b_{5} = b_{3} \cdot q^{2}\]
\[12 \cdot q^{2} = 48\]
\[q^{2} = 48\ :12\]
\[q1^{2} = 4\]
\[q = 2.\]