Two thin lenses of focal lengths $20 \ cm$ and $25 \ cm$ are placed in contact. The effective power of the combination is (in $D$)

$A$ converging lens of focal length $30 \,cm$ is placed in contact with another converging lens of unknown focal length. Then,the possible value for the focal length of the combination is ......... $cm$.

The power of an achromatic convergent lens combination consisting of two lenses is $+ 2D$. The power of the convex lens is $+ 5D$. The ratio of the dispersive power of the convex lens to the concave lens will be:

$A$ convex lens of focal length $30 \ cm$ and a concave lens of $10 \ cm$ focal length are placed so as to have the same axis. If a parallel beam of light falling on the convex lens leaves the concave lens as a parallel beam,then the distance between the two lenses will be.......$cm$.

An eye specialist prescribes spectacles having a combination of a convex lens of focal length $40\, cm$ in contact with a concave lens of focal length $25\, cm$. The power of this lens combination in diopters 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 $cm$.