A 10 mm pin in front of a concave mirror forms a real, inverted 5 mm image at 30 cm in front of the mirror. The focal length of the mirror is

$m=h'/h=-5/10=-0.5=-v/u$ with $v=-30$, so $u=-60$ cm. $\frac1f=\frac1v+\frac1u=-\frac1{30}-\frac1{60}=-\frac{3}{60}\Rightarrow f=-20$ cm. Magnitude 20 cm.