\[b_{1} = 5,4;\ \ S_{n} = 9;\]
\[S = \frac{b_{1}}{1 - q}\]
\[9 = \frac{5,4}{1 - q}\]
\[9 \cdot (1 - q) = 5,4\]
\[1 - q = 5,4\ :9\]
\[1 - q = 0,6\]
\[q = 1 - 0,6\]
\[q = 0,4.\]
\[Ответ:q = 0,4.\]