Find out equivalent focal length of given lens combination

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 :

The ratio of powers of a thin convex and thin concave lens is 3/2 and equivalent focal length of their combination is 30 cm. Then their focal lengths respectively are

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 thin convex lens is put in contact with a thin concave lens of equal focal length. The effective power and focal length of the lens combination respectively are

A converging and a diverging lens of equal focal lengths are placed co-axially in contact. Find the power and the focal length of the combination.

Write expression for the focal length and power of a lens 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