In which optical instruments is a combination of lenses used?

$A$ convex lens and a concave lens,each having the same focal length of $25\, cm$,are put in contact to form a combination of lenses. The power in diopters of the combination is:

$A$ glass convex lens of focal length $0.1 \ m$ is cut into two equal parts along its axis. The ratio of the focal length of the new lenses is:

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

Two thin convex lenses are kept in contact coaxially. If the focal length of the combination of the lenses is $4 \ cm$ and the sum of the focal lengths of the two lenses is $18 \ cm$,then the focal length of the lens of lower power is (in $cm$)

The size of the image of an object, which is at infinity, as formed by a convex lens of focal length $30\,cm$ is $2\,cm$. If a concave lens of focal length $20\,cm$ is placed between the convex lens and the image at a distance of $26\,cm$ from the convex lens, calculate the new size of the image. (in $cm$)