Answer
$1,953,120$
Work Step by Step
$\dfrac{a_{n+1}}{a_n}=\dfrac{4(5^{n+1})}{4(5^n)}=5$
Since the ratio between consecutive terms is constant, the given sequence is geometric.
The first term is $a_1=4(5^1)=20$.
The common ratio is $r=5$.
The formula for the sum of the first $n$ terms is:
$S_{n}=\frac{a_{1}(1-r^{n})}{1-r}$
$S_{8}=\frac{20(1-5^{8})}{1-5}=1,953,120$
