Two thin lenses of power +5 D and -2.5 D are in contact. What is the focal length of the combination ?

Two thin lenses of power + 6 D and - 2 D are in contact. What is the focal length of the combination ?

Two thin lenses of power + 6 D and - 2 D are in contact. What is 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 thin lenses of power +6 D and -2 D are in contact. What is the focal length of the combination ?

Two thin lenses of power +4 D and -2 D are in contact. What is the focal length of the combination ?

Two thin lenses of power +4 D and -2 D are in contact. What is the focal length of the combination ?

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

Two thin lenses of power 2D and -4D are kept in contact. What is the focal length of the combination in cm? (write the answer only)