The focal length of a convex lens is maximum for which of the following?

The distance between an object and the screen is $80 \, cm$. $A$ lens produces an image on the screen when the lens is placed at either of the positions $20 \, cm$ apart. The focal length of the lens is ....... $cm$.

The focal length of a convex lens is $30 \ cm$ and the size of the image is a quarter of the object,then the object distance is ........ $cm$.

An object approaches a convergent lens from the left of the lens with a uniform speed $5 \ m/s$ and stops at the focus. The image:

Two point sources $S_1$ and $S_2$ are $24 \ cm$ apart. Where should a convex lens of focal length $9 \ cm$ be placed in between them,so that the images of both sources are formed at the same place (in $cm$)?

At what distance from a biconvex lens of focal length $F$ must an object be placed so that the distance between the object and its real image is minimal?