Two lenses of power $6 \, D$ and $-2 \, D$ are placed in contact to form a single lens. The focal length of the combination is:

Two lenses of powers $-1.75 D$ and $+2.25 D$ are placed in contact. The focal length of the combination is (in $\,cm$)

$A$ plano-convex lens of material of refractive index $\mu_1$ exactly fits into a plano-concave lens of material of refractive index $\mu_2$. If $R$ is the radius of curvature of the curved surfaces of the lenses and the plane surfaces of the lenses are parallel,the focal length of the combination is:

$A$ real image of an object is formed at a distance of $20 \, cm$ from a lens. On putting another identical lens in contact with it,the image is shifted $10 \, cm$ towards the combination. The power of the lens is (in $, D$)

An optician makes 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 combination of lenses in dioptre is:

Find the distance of the final image from the object $O$,formed by the combination of lenses shown in the figure (in $cm$):