How do magnification and focal length change for a combination of thin lenses ?

Obtain an expression for focal length of a combination of thin lenses in contact.

We combined a convex lens of focal length f_(1) and concave lens of focal lengths f_(2) and their combined focal length was F . The combination of these lenses will behave like a concave lens, if

Magnification by a lens of an object at distance 10 cm from it is -2. now a seconnd lens is placed exactly at the same positon where first was kept, without changing the distance between object and lens. The magnification by this second lens is -3. What is the focal length of the combination when both lenses are in contact ?

A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. The focal length of this combination of lenses will be:

A thin convex lens of focal length 10 cm is placed in contact with a concave lens of same material and of same focal length. The focal length of combination will be

Find the lateral magnification produced by the combination of lenses shown in Figure.

When two thin lenses are kept in contanct, the focal length of the combination is

Two thin lenses, aone convex of focal length 10 cm and other concave lens are kept in contact to form a composite lens of focal length 13 cm . If the combination acts as a converging lense the focal length of second lens is