Two lenses of focal lengths -20cm and +10cm are put in contact. Find power of the combination (A) -1D (B) -2D (C) +5D (D) +2D

Two thin lenses of focal lengths 20cm and 25cm are placed in contact. The effective power of the combination is

Two thin lenses of focal lengths 20 cm and 25 cm are placed in a contact. The effective power of the combination is

Two lenses with focal length 20 cm and - 10 cm are placed together. Determine the power of combination.

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

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

Three lenses lenses of focal length 20 cm each are kept in contact with each other . Find the power of their combination.

Two thin lenses of focal lengths 20 cm and -20 cm are placed in contact with other. The combination has a focal length equal to