Two lenses of power 12 and -2 dioptre are placed in contact. The focal length of the combination is

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

Q. A convex lens of focal length 50 cm, a concave lens of focal length 50 cm and another concave lens of focal lens 20 cm are placed in contact. The power of this combination in dioptres will be:

Q. A convex lens of power $+6$ dioptre is placed in contact with a concave lens of power $-4$ dioptres. What will be the nature and focal length of this combination?

Q. Two convex lenses with power 2 dioptre are kept in contact with each other. The focal length of the combined lens system is