There is a thin biconvex lens having a focal length $20 \mathrm{cm}$. If an object is moved to $50 \mathrm{cm}$ from a distance of $25 \mathrm{cm}$ in front of it, then magnification of its image change from $m_{25}$ to $m_{50}$ . Find $m_{25}:m_{50 }=?$