A convex mirror forms an image whose area is one-ninth of the object area. When the object is moved 20 cm toward the mirror, image area becomes one-fourth of object area. Find the focal length magnitude in centimetres.
A convex mirror forms an image whose area is one-ninth of the object area. When the object is moved 20 cm toward the mirror, image area becomes one-fourth of object area. Find the focal length magnitude in centimetres.