User:IssaRice/Pareto distribution

I still don't know how this is derived, but here's how to make sense of the 80/20 rule:

What does it mean to say that the top 20% own 80% of the wealth? It means that ${\frac {\int _{c}^{\infty }xf(x)\,dx}{\int _{x_{m}}^{\infty }xf(x)\,dx}}=0.80$ where we choose the cutoff c according to $\int _{c}\infty f(x)\,dx=0.20$.