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

$A$ 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 $10 \ cm$ 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.......$cm$.

$A$ bi-convex lens is formed with two thin plano-convex lenses as shown in the figure. The refractive index $n$ of the first lens is $1.5$ and that of the second lens is $1.2$. Both the curved surfaces are of the same radius of curvature $R = 14 \, cm$. For this bi-convex lens,for an object distance of $40 \, cm$,the image distance will be.......$cm$

An object $AB$ is placed $15 \text{ cm}$ on the left of a convex lens $P$ of focal length $10 \text{ cm}$. Another convex lens $Q$ is now placed $15 \text{ cm}$ to the right of lens $P$. If the focal length of lens $Q$ is $15 \text{ cm}$, the final image is . . . . . . .

$A$ lens of power $+2 \text{ D}$ is placed in contact with a lens of power $-1 \text{ D}$. The combination will behave like: