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 $2$?

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 $D$)

Three thin lenses are combined by placing them in contact with each other to get more magnification in an optical instrument. Each lens has a focal length of $3 \,cm$. If the least distance of distinct vision is taken as $25 \,cm$,the total magnification of the lens combination in normal adjustment is

Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is

Two lenses of power $-15 \ D$ and $+5 \ D$ are in contact with each other. The focal length of the combination is ....... $cm$.

$A$ convex lens of focal length $30 \ cm$ and a concave lens of $10 \ cm$ focal length are placed so as to have the same axis. If a parallel beam of light falling on the convex lens leaves the concave lens as a parallel beam,then the distance between the two lenses will be.......$cm$.