A plano-convex lens P and a concavo-convex lens Q are in contact as shown in figure. The refractive index o fthe material of the lens P and Q is `1.8 and 1.2` respectively. The radius of curvature of the concave surface of the lens Q is double the radius of curvature of the convex surface. The convex surface of Q is silvered.
An object is placed on the placed on the principal axis at a distance `10 cm` form the plane surface. The image is formed at a distance `40 cm` from the plane surfaces on the same side. The focal length of the system is
