A bright point object is placed at a distance of 15 cm in front of a fixed convex lens of focal length 10 cm (on its principal axis). A concavo convex lens of refractive index 1.5 with radii of curvature 60cm and 30cm can move coaxially behind the convex lens. The convex surface of the concavo – convex lens is silvered. It is observed that at two
positions P and Q of the silvered lens, the final image coincides with the object. If the distance between P and Q is [15+x]cm, find the value of x.