Answer
$a_8=640$
Work Step by Step
We have to find the $8$th term of the geometric sequence:
$5, 10, 20, 40, . . .$
For the general term $a_n$ we have
$a_n=a_1\cdot r^{n-1}$.
In our case
$r=\frac{a_2}{a_1}=\frac{10}{5}=2$.
So
$a_8=5\cdot 2^{8-1}=5\cdot2^7=5\cdot128=640$
