A convex lens of focal length \( 20 \, \text{cm} \) is placed in front of a convex mirror with principal axis coinciding each other. The distance between the lens and mirror is \( 10 \, \text{cm} \). A point object is placed on principal axis at a distance of \( 60 \, \text{cm} \) from the convex lens. The image formed by combination coincides the object itself. The focal length of the convex mirror is ______________ \( \text{cm} \).
