Answer
$105$
Work Step by Step
Using the given general term, $
a_n=5(4)^{n-1}
,$ then,
\begin{array}{l}
a_1=5(4)^{1-1}\\
a_1=5(1)\\
a_1=5
,\\\\
a_2=5(4)^{2-1}\\
a_2=5(4)\\
a_2=20
,\\\\
a_3=5(4)^{3-1}\\
a_3=5(16)\\
a_3=80
.\end{array}
Hence, $
S_3=5+20+80
=
105
.$
