Two lenses of power $-15 \, D$ and $5 \, D$ are placed in contact with each other. The focal length of this combination will be ....... $cm$.

$A$ convex lens of focal length $10 \, cm$ is placed in contact with a concave lens. The focal length of the combination is numerically equal to that of the concave lens. The focal length of the concave lens is ............ $cm$.

Two lenses of focal lengths $f_1$ and $f_2$ are placed in contact,and the image is formed at a distance of $60 \, cm$. When the lenses are separated by a distance of $10 \, cm$,the image is formed at a distance of $30 \, cm$. What are the values of $f_1$ and $f_2$?

An effective power of a combination of $5$ identical convex lenses which are kept in contact along the principal axis is $25 \ D$. Focal length of each of the convex lens is: (in $cm$)

Two lenses of powers $-1.75 D$ and $+2.25 D$ are placed in contact. The focal length of the combination is (in $\,cm$)

An achromatic convergent lens of focal length $20 \, cm$ is made of two lenses (in contact) of materials having dispersive powers in the ratio of $1 : 2$ and having focal lengths $f_1$ and $f_2$. Which of the following is true?