A concave mirror of focal length $20 \mathrm{cm}$ and a convex lens of focal length $10 \mathrm{cm}$ are kept with their optic axes parallel but separated by$0.5 \mathrm{cm}$ as shown in the figure. The distance between the lens and mirror is $10 \mathrm{cm}$. An object of height $3 \mathrm{mm}$ is placed on the optic axis of lens at a distance $15 \mathrm{cm}$ from the lens. Find the length of the image formed by the mirror in$\mathrm{mm}$.