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 $A$ and a concave lens of focal length $B$ are placed in contact with each other. The focal length of this combination is ......

$A$ convex lens and a concave lens, each with a focal length of $4 \,cm$, are separated by a distance of $6 \,cm$ along their axis. An object is placed $8 \,cm$ before the convex lens. The distance between the object and its final image is (in $\,cm$)

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

$A$ convex lens of focal length $30 \ cm$ is placed in contact with a concave lens of focal length $20 \ cm$. An object is placed at $20 \ cm$ to the left of this lens system. The distance of the image from the lens in $cm$ is . . . . . .

What is the position and nature of the image formed by the lens combination shown in the figure? ($f_1, f_2$ are focal lengths)