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 $\frac{\underset{c}{\overset{\mathrm{\infty }}{\int }}xf(x)dx}{\underset{x_m}{\overset{\mathrm{\infty }}{\int }}xf(x)dx}=0.80$ where we choose the cutoff c according to $\underset{c}{\overset{\mathrm{\infty }}{\int }}f(x)dx=0.20$.