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 20 cm and -20 cm are placed in contact with other. The combination has a focal length equal to

A convex lens of focal lenth 30 cm and a concave lens of focal length 60 cm are placed in contact.Find the power of the combination.

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 lenths + 10 cm and - 5 cm are kept in contact.What is the power of combination.

Two lenses of focal lengths +10 cm and -15 cm are put in contact to form an achromatic combination. The ratio of their dispersive power is :

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