So I was working on a probability question and then this expression came up.
When I consulted the answers, I struggled to understand exactly how I would get from one expression to the other myself.
Substituting a constant such as
let $n=5$ makes it a bit clearer how they got from one expression to the other.
But is there a simple, yet general explanation of these expressions.
I am hoping the community could give some insight into how I should have approached this problem and similar ones in future.