Answer
$S_{20}=-410$
Work Step by Step
The sequence $
8,5,2,...
$ is an arithmetic sequence with $a_1=8$ and $d=-3$. Using $S_n=\dfrac{n}{2}(2a_1+(n-1)d)$, then,
\begin{array}{l}
S_{20}=\dfrac{20}{2}(2\cdot8+(20-1)(-3))\\
S_{20}=10(16+(-57))\\
S_{20}=10(-41)\\
S_{20}=-410
.\end{array}
