\[\frac{1}{54},\ \frac{1}{18},\ \frac{1}{6},\ \ldots\ \ \]
\[q = \frac{1}{18} \cdot \frac{54}{1} = 3;\]
\[S_{6} = \frac{\frac{1}{54} \cdot (729 - 1)}{2} = \frac{728}{108} =\]
\[= \frac{182}{27} = 6\frac{20}{27}.\]
\[Ответ:\ \ 6\frac{20}{27}.\]