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

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

Two lenses f powers-1.5 and +2.75 D are kept in contact.Find the focal length of the combination.

What is the focal length of a plane mirror?

Two lenses of powers+15D and -5D are in contact with each other. What is the focal length of combination?

What is the focal length of a plane mirror ?

Two thin lenses of focal lenths + 10 cm and - 5 cm are kept in contact.What is the focal length and