An effective power of a combination of $5$ identical convex lenses which are kept in contact along the principal axis is $25 \ D$. Focal length of each of the convex lens is: (in $cm$)

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

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

An object is placed $12 \ cm$ to the left of a converging lens of focal length $8 \ cm$. Another converging lens of $6 \ cm$ focal length is placed at a distance of $30 \ cm$ to the right of the first lens. The second lens will produce

The size of the image of an object at infinity, 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, what is the new size of the image (in $\,cm$)?

Find the position of the final image with respect to the rightmost lens formed by the lens combination shown in the figure.