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

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

Two lenses of power +15 D and -5 D are in contact with each other forming a comvination lens. what is the focal length of this combination?

Two lenses of power +12 D and -2D are combined together.What is their equivalent focal length?

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

If two lenses of power +1.5 D and +1.0 D are placed in contact ,then the effective power of combination will be

Two thin converging lenses of focal lenths 15 cm and 30 cm are held in contact with each other.Calculate power and focal length of the combination.