Two lenses of power 15D and -3D are placed in contact. The focal length of the combination is a

Two lenses of power 15 and - 3 dioptre are placed in contact. The focal length of the combination is

Two lenses of power +2D and -5D are kept in contact. The focal length of the combination is

Two coaxial lenses of power + 4D and - 2D are placed in contact. The focal length of combination is

Two lenses of powers + 1.5 D and - 1 D are kept in contact. The focal length of the combination is

Two lenses of powers +5D and - 3D are in contact: Find the focal length of the combination.

Two thin lenses of power + 5D and - 3D are in contact. What is the focal length of the combination ?

Two lenses of powers +5D and 3D are in contact.Find the focal length of combination.

Two lenses of power 15.5 D and -5.5 D are placed in contact.What is teh focal length of the combbination?