Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact. The focal length of this combination is

In the figure shown here,what is the equivalent focal length (in $cm$) of the combination of lenses (Assume that all layers are thin)?

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:

For a combination of two convex lenses of focal lengths $f_1$ and $f_2$ to act as a glass slab,the distance of separation between them is

Two identical thin biconvex lenses of focal length $15 \, cm$ and refractive index $1.5$ are in contact with each other. The space between the lenses is filled with a liquid of refractive index $1.25$. The focal length of the combination is $cm$.

$A$ convex lens has $9 \ cm$ focal length and a concave lens has $-18 \ cm$ focal length. The focal length of the combination in contact will be.....$cm$