Is power negative or positive for a concave lens?

Two convex lenses of focal lengths $f_1$ and $f_2$ form images with magnification $m_1$ and $m_2$, when used individually for an object kept at the same distance from the lenses. Then, $f_1 / f_2$ is

The dispersive powers of the glasses of lenses used in an achromatic pair are in the ratio $5 : 3$. If the focal length of the concave lens is $15 \ cm$,then the nature and focal length of the other lens would be:

An object is moving with a velocity $V_O = 4 \text{ cm/sec}$ towards a convex lens of focal length $f = 10 \text{ cm}$. The lens is also moving with a velocity $V_L = 2 \text{ cm/sec}$ in the same direction. At the instant when the object is at a distance of $30 \text{ cm}$ from the lens,find the velocity of the image.

$A$ glass convex lens $(\mu_g = 1.5)$ has a focal length of $8\, cm$ in air. What will be its focal length when it is immersed in water (in $, cm$)? $(\mu_w = 1.33)$

$A$ bi-concave glass lens having refractive index $1.5$ has both surfaces of same radius of curvature $R$. On immersion in a medium of refractive index $1.75$,it will behave as a