Free.
Assume that you are living in a world that has a population of exactly 8589934591 (including you). There is a disease that everyone has including you. You cure yourself of it and find out how to cure other people. Once someone is cured they can cure others. Thus you decide that you will cure 2 people and then they will cure 2 people each and then the cured will then cure 2 people each etc. Think of this process in ‘cure steps’ i.e. you make the first step (you cure 2 people), then they make the second step (by curing 2 people each) etc.
How many such ‘cure steps’ must be taken in order for the whole population to be cured?
HINT: What do you know about the geometric series?