jmc

We have the system of equations: $$\begin{array}{rl}x+y& =399\\ \frac{x}{y}& =0.9\end{array}$$ From the second equation, multiplying both sides by $y$ gives $x=.9y$. Next, substituting the second equation into the first to eliminate $x$ gives $.9y+y=399$, or $y=210$. Plugging in this value into the first equation in the original system of equations gives $x+210=399$ or $x=189$. Thus, $y-x=210-189=\boxed{21}$.