A convex lens of focal length 'f' is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is

A convex lens of focal length f is put in contact with a concave lens of the same focal length. The equivalent focal length of the combination is :

A convex lens of focal length cc"f" is put in contact with a concave lens of the same focal length. The equivalent focal length of the combination is :

A convex lens of focal length 10.0 cm is placed in contact with a convex lens of 15.0 cm focal length. What is the focal length of the combination ?

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

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 convax lens of focal length 10.0cm is placed in contact with a convex lens of 15.0cm focal length ,What is the focal length of the combination.

A convex lens of focal length 10 cm is placed in contact with a concave lens of focal length 20 cm. What is the nature and focal length of the combination ?