Answer
$\displaystyle\sum_{i=1}^{5}\left[80\left( \dfrac{1}{4} \right)^{i-1}\right]
$
Work Step by Step
Using the summation notation, the series $
80+20+5+\dfrac{5}{4}, \dfrac{5}{16}
$ is equivalent to
\begin{array}{l}
\displaystyle\sum_{i=1}^{5}\left[80\left( \dfrac{1}{4} \right)^{i-1}\right]
.\end{array}