When will a concave lens behave like a convex lens and when will a convex lens behave like a concave lens?

Q. When the plane surface of a plano-convex lens of refractive index $1.5$ is silvered, it behaves like a concave mirror with $f=30cm$.When its convex surface is silvered, it will behave like a concave mirror of focal length:

Q. A plano-convex lens when silvered on the plane side behaves like a concave mirror of focal length $60cm$. However, when silvered on the convex side, it behaves like a concave mirror of focal length $20cm$.Then, the refractive index of the lens is

Q. A convex lens of focal length $f_1$ is combined with a concave lens of focal length $f_2$ and their combined focal length is observed to be $f$. The combination of these lenses will behave like a concave lens if

Q. A concave lens has been placed in an optically rarer medium. The lens will behave like