If a convex lens of focal length 80 cm and a concave lens of focal length 40 cm are combined together. What will be their resulting power?

A convex lens of focal length f_1 and a concave lens of focal length f_2 are placed in contact. The focal length of the combination is f. Then

Two thin lenses are in contact with each other. One of these is a concave lens of focal length 20 cm and the other is a convex lens of same focal length of the combination and state how it behaves.