Find the equivalent focal length of a combination of two convex lenses in contact,each having a focal length of $30 \ cm$. (in $cm$)

Two lenses in contact made of materials with dispersive powers in the ratio $2 : 1$ behave as an achromatic lens of focal length $10\,cm$. The individual focal lengths of the lenses are:

Three thin lenses are combined by placing them in contact with each other to get more magnification in an optical instrument. Each lens has a focal length of $3 \,cm$. If the least distance of distinct vision is taken as $25 \,cm$, the total magnification of the lens combination in normal adjustment is

$A$ convex lens of focal length $40 \, cm$ and a concave lens of focal length $25 \, cm$ are placed in contact. The power of the combination is .......... $D$.

$A$ convex lens of focal length $40 \; cm$ is in contact with a concave lens of focal length $25 \; cm$. The power of the combination is:

Two thin equiconvex lenses each of focal length $0.2 \,m$ are placed coaxially with their optic centres $0.5 \,m$ apart. Then the focal length of the combination is (in $\,m$)