If two $+5D$ lenses are mounted at some distance apart,the equivalent power will always be negative if the distance is

$A$ real image of an object is formed at a distance of $20 \, cm$ from a lens. On putting another identical lens in contact with it,the image is shifted $10 \, cm$ towards the combination. The power of the lens is (in $, D$)

In a certain camera,a combination of four similar thin convex lenses are arranged axially in contact. Then the power of the combination and the total magnification in comparison to the power $(p)$ and magnification $(m)$ for each lens will be,respectively

$A$ convex lens of power $+6 D$ is placed in contact with a concave lens of power $-4 D$. What is the nature and focal length of the combination?

An achromatic combination of lenses produces

Two planoconvex lenses each of focal length $10 \, cm$ and refractive index $3/2$ are placed as shown. In the space left, water $(R.I = 4/3)$ is filled. The whole arrangement is in air. The optical power of the system is (in diopters):