The radius of curvature of concave mirror is 20 cm. Determine the focal length of mirror.

Given that: 
Radius of curvature, R = - 20 cm 
here, R is taken as negative since the mirror is concave.
Now, we know that R = 2f
Hence, focal length, $f = \frac{R}{2}$
or  $f = \frac{-20 cm}{2}$
or $f = -10 cm$
Therefore, the focal length of the concave mirror is 10 cm.