Two lenses of power $+10D$ and $-5D$ are placed in contact. At what distance (in $cm$) should an object be held from the lens combination to obtain a virtual image of magnification $2$?

$A$ biconvex glass lens has a focal length $f$ and power $P$. It is cut into two symmetric parts along its principal axis. If these two parts are joined as shown in figure $(b)$,what is the new power of the combination?

$A$ magnifying glass is made of a combination of lenses with power $+20\,D$ and $-4\,D$. If the least distance of distinct vision is $25\,cm$,calculate the size of an image of an object $2\,cm$ high seen through the magnifying glass. (in $,cm$)

Two lenses of power $+12 \ D$ and $-2 \ D$ are placed in contact. What will be the focal length of the combination in $cm$?

Two thin convex lenses of focal lengths $30 \ cm$ and $10 \ cm$ are placed coaxially,$1 \ cm$ apart. The power of this combination is (in $D$)

Obtain the equation of power and magnification for a combination of lenses.