When an object is placed at a distance of 60cm from a convex spherical mirror, the magnification produced is 1/2. Where should the object placed to get magnification of 1/3?

Q. When an object is placed at a distance of $25cm$ from a spherical mirror, the magnification is $m_1$. When the object is moved $15cm$ away from mirror with respect to the earlier position, then magnification becomes $m_2$. If $\frac{m_1}{m_2}=4$, then the focal length of the mirror is :