Answer
See below
Work Step by Step
An infinite geometric series converges if and only if $|r|\lt1$, where $r$ is the common ratio. If it converges, then it equals $\frac{a_1}{1-r}$ where $a_1$ is the first term.
Here only $r=0.75$ has an absolute value less than $1$, thus the series that can be summed up is $a_n=20(0.75)^{n-1}$
