Two lenses of focal lengths -20cm and +10cm are put in contact.Find power of the 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

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 of focal lengths 20 cm and - 40 cm are held in contact. The image of an object at infinity will be formed by the combination at

Two thin concave lenses of focal lengths 10 cm and 30 cm and a thin convex lens of focal length 30 cm are kept in contact. What will be the power of lens combination ?

If two convex lenses of focal lengths 10 cm and 5 cm are kept in contact with each other , what is their combined focal length ?

A convex lens of focal length 40 cm a concave lens of focal length 40 and a concave lens of focal length 15 cm are placed in contact. The power of this combination of are placed in contact. The power of this combination in diopters is

A convex lens of power 2 D and a concave lens of focal length 40cm are kept in contact, find (a) Power of combination , (b) Equivalent focal length

Two thin lenses of focal length f_(1) and f_(2) are in contact and coaxial. The power of the combination is