9000150504

The object $y$ is projected using a lens with foci at $F$ and $F^{\prime }$. The focal length of the lens (the distance from the focus to the lens) $f=20\mathrm{cm}$. The distance from the object $y$ to the lens $a=60\mathrm{cm}$. Find the distance from the lens to the image $y^{\prime }$.