An object is placed at a distance of 30 cm in front of an unknown mirror. The mirror forms an image at 20 cm behind itself. Find out the focal length and type of the mirror.

The correct option is D 60 cm, convex mirror
Using sign conventions, we can write:
Object distance, u = -30 cm
Image distance, v = +20 cm
Using the mirror formula,
$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$
$\frac{1}{f}=\frac{1}{20}+\frac{1}{-30}$
$f=60cm$
The focal length is positive; hence, it is a convex mirror.