jmc

Call the constant $r$. To get from $2$ to $\frac{1}{2}$, we multiplied by $r$, so $2r=\frac{1}{2}$, or $r=\frac{1}{4}$. To get from 128 to $x$, we multiply by $\frac{1}{4}$, so $x=128\cdot \frac{1}{4}=32$. To get from 32 to $y$, we multiply by $\frac{1}{4}$, so $y=32\cdot \frac{1}{4}=8$. Thus, $x+y=32+8=\boxed{40}$.