A double convex glass lens $\left(R_1=R_2=10 \mathrm{cm}\right)$ having focal length equal to the focal length of a concave mirror. The radius of curvature of the concave mirror is $\left({\mu }_g=1.5\right)$

Figure shows two rays $A$ and $B$ being reflected by a mirror and going as $A^{\text{'}}$ and $B^{\text{'}}$. The mirror

A slab of glass, of thickness $6 \mathrm{cm}$ and refractive index $\mu =1.5$ is placed in front of a concave mirror, as shown in the figure. If the radius of curvature of the mirror is $40 \mathrm{cm}$ and the reflected image coincides with the object, then the distance of the object from the mirror is