At what distance in front of a concave mirror of focal length $30\; cm$,should an object be placed (in $cm$) so that its real image of size three times that of the object is obtained?

For a real object,which of the following can produce a real image?

The radius of curvature of a concave mirror is $40 \ cm$ and the size of the image is twice that of the object. The object distance is $... \ cm$.

How is it convenient to obtain an image by reflection from a spherical mirror?

If a parallel beam of light is incident on spherical mirrors, then after reflection, they pass through the focal point. If $F$ is the focal length and $R$ is the radius of curvature of the mirror, then:

$A$ thin rod of $5 \, cm$ length is kept along the axis of a concave mirror of $10 \, cm$ focal length such that its image is real and magnified. If one end of the rod is at a distance of $2f$ from the mirror,what will be its magnification?