Answer
The series $\sum_{n=5}^{\infty} (4/5)^{-n}$ diverges.
Work Step by Step
We have
$$ \lim_{n\to \infty} (4/5)^{-n}=\lim_{n\to \infty} (5/4)^{n}=\infty.$$
Where we used the fact that $5/4\gt 1$. Hence, the series $\sum_{n=5}^{\infty} (4/5)^{-n}$ diverges.
