Answer
$13.5$
Work Step by Step
We can see that the ratio of subsequent terms is $\frac{1}{3}$, thus $r=\frac{1}{3}$.
The sum of an infinte geometric series can be dividing the first term($a_1$) by the difference of $1$ and the common ratio ($r$). So $S=\frac{a_1}{1-r}$.
Hence here $S=\frac{9}{1-\frac{1}{3}}=13.5$.
