A system consists of a convex lens and a concave mirror placed at a distance of x from the lens. Focal length $f_1$ of convex lens in 10 cm and focal length $f_2$ of mirror is 60 cm. If an object is placed at a distance 20 cm from the lens and it is found that the final image coincides with the object position, then what must be the distance 'x'

Question

$(a)\;60\;cm \\ (b)\;40\;cm \\ (c)\;20\;cm \\ (d)\;10\;cm$