Answer
$$\frac{{81}}{2}$$
Work Step by Step
$$\eqalign{
& 27 + 9 + 3 + 1 + \cdots \cr
& {\text{Sum of a Geometric Series }}\left( {{\text{see page 635}}} \right) \cr
& {\text{The infinite geometric series}} \cr
& a + ar + a{r^2} + a{r^3} + \cdots \cr
& {\text{converges}}{\text{, if }}r{\text{ is in }}\left( { - 1,1} \right),{\text{ to the sum }}\frac{a}{{1 - r}}.{\text{ And diverges if }}r{\text{ is not in }}\left( { - 1,1} \right) \cr
& {\text{then this is a geometric series}}{\text{, with }}a = {a_1} = 27 \cr
& r = \frac{9}{{27}} = \frac{1}{3} \cr
& {\text{Since }}r{\text{ is in the interval }}\left( { - 1,1} \right),{\text{ the series converges and has a sum }}\frac{a}{{1 - r}} \cr
& \frac{a}{{1 - r}} = \frac{{27}}{{1 - 1/3}} \cr
& {\text{simplifying}} \cr
& \frac{a}{{1 - r}} = \frac{{81}}{2} \cr} $$