I am looking at my teacher's solutions for a Statistics problem and he's made the following equivalence. I am not sure why he did this, if it helps, we're doing probability with the geometric distribution.

$$\sum\_{x=21}^\infty (.2)(.8)^{x-1} = .2\frac{.8^{21-1}}{1-.8}$$

If needed, the geometric distribution formula is the following:

$$p(y)=p(1-p)^{(y-1)}$$