jmc

We have the system of equations:

$$\begin{array}{rl}x+y& =153\\ \frac{x}{y}& =0.7\end{array}$$

From the second equation, multiplying both sides by $y$ gives $x=.7y$. Next, substituting the second equation into the first to eliminate $x$ gives $.7y+y=153$, or $y=90$. Plugging in this value into the first equation in the original system of equations gives $x+90=153$ or $x=63$. Thus, $y-x=90-63=\boxed{27}$.