18. B From the equation $\frac{x}{y}=\frac{2}{3}$ we deduce that $3 x=2 y$. We are given that $x+y=\frac{2}{3}$, so $3 x+3 y=2$. Substituting for $3 x$ gives $2 y+3 y=2$, so $y=\frac{2}{5}$. Therefore $x-y=x+y-2 y=\frac{2}{3}-\frac{4}{5}=\frac{10}{15}-\frac{12}{15}=-\frac{2}{15}$.
18. Two numbers $x$ and $y$ are such that $x+y=\frac{2}{3}$ and $\frac{x}{y}=\frac{2}{3}$. What is the value of $x-y$ ?
A $-\frac{2}{3}$
B $-\frac{2}{15}$
C $\frac{2}{25}$
D $\frac{2}{5}$
$\mathrm{E} \frac{2}{3}$