Out of convex mirror and concave mirror, whose focus is situated behind the mirror?

Explain how to read the following message which was found on some blotting paper:

Name the spherical mirror which has:

(a) Virtual principal focus.

(b) Real principal focus.

Find the focal length of a concave mirror whose radius of curvature is 30 cm.

If the focal length of a convex mirror is 28 cm, what is its radius of curvature?