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 $\frac{2}{5}$
$\mathrm{E} \frac{2}{3}$