$A$ 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 of focal length $40 \; cm$ is in contact with a concave lens of focal length $25 \; cm$. The power of the combination is:

$A$ convex lens of focal length $30\, cm$ and a concave lens of focal length $10\, cm$ are placed coaxially at a distance $d$ apart. Parallel rays are incident on the convex lens. The rays emerging from the concave lens are also parallel. Then $d = $ . . . . . . $cm$.

Two lenses of power $+10 \, D$ and $-5 \, D$ are placed in contact. Where should an object be held from the lens so as to obtain a virtual image of magnification $2$ (in $, cm$)?

$A$ parallel beam of light is incident on a system of two convex lenses with focal lengths $f_1 = 20 \, cm$ and $f_2 = 10 \, cm$. What should be the distance between the two lenses so that the rays,after refraction from both lenses,emerge as a parallel beam? (in $cm$)

$A$ plano-convex lens of refractive index $\mu_1$ fits exactly into a plano-concave lens of refractive index $\mu_2$. Their plane surfaces are parallel to each other. $R$ is the radius of curvature of the curved surface of the lenses. The focal length of the combination is: