Let $x_1,x_2, ... , x_n$ be positive integers,Asumme that in their decimal representations no $x_i$ "prolongs" $x_j$.For instance , $123$ prolongs $12$ , $459$ prolongs $4$ , but $124$ does not prolog $123$. Prove that : $\frac {1}{x_1}+\frac {1}{x_2}+...+\frac {1}{x_n} < 3$.