Two convex lenses of focal length $20\,cm$ each are placed coaxially with a separation of $60\,cm$ between them. The image of the distant object formed by the combination is at $...........\,cm$ from the first lens.

$A$ convex lens $A$ of focal length $20 \,cm$ and a concave lens $B$ of focal length $56 \,cm$ are kept along the same axis with a distance $d$ between them. If a parallel beam of light falling on $A$ leaves $B$ as a parallel beam, then the magnitude of distance $d$ (in $cm$) is

$A$ convex lens $A$ of focal length $20 \, cm$ and a concave lens $B$ of focal length $5 \, cm$ are kept along the same axis with a distance $d$ between them. If a parallel beam of light falling on $A$ leaves $B$ as a parallel beam,then the distance $d$ in $cm$ will be:

$A$ combination of two thin lenses with focal lengths $f_1$ and $f_2$ respectively forms an image of a distant object at a distance of $60 \ cm$ when the lenses are in contact. The position of this image shifts by $30 \ cm$ towards the combination when the two lenses are separated by $10 \ cm$. The corresponding values of $f_1$ and $f_2$ are:

$A$ convex lens of focal length $40 \, cm$ and a concave lens of focal length $25 \, cm$ are placed in contact. The power of the combination is .......... $D$.

Two lenses are placed in contact with each other and the focal length of the combination is $80 \ cm$. If the focal length of one lens is $20 \ cm$,then the power of the other lens will be: (in $D$)