Two lenses of power $6 \, D$ and $-2 \, D$ are placed in contact to form a single lens. The focal length of the combination is:

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.

Find the position of the final image with respect to the rightmost lens formed by the lens combination shown in the figure.

Two thin lenses of focal lengths $20 \ cm$ and $25 \ cm$ are placed in contact. The effective power of the combination is (in $D$)

$A$ concave lens and a convex lens are arranged as shown in the figure. Find the position of the final image.

$A$ biconvex lens is formed by using two thin planoconvex lenses,as shown in the figure. The refractive index and radius of curved surfaces are also mentioned in the figure. When an object is placed on the left side of the lens at a distance of $30 \ cm$ from the biconvex lens,the magnification of the image will be: