Answer
+40 cm
Work Step by Step
Refractive index of glass $n=1.5$
Radius of curvature of convex side $r_1=+20\ cm$
Radius of curvature of flat side $r_2=∞$
To find the focal length of the length f, we use the formula
$\frac{1}{f}=(n-1) (\frac{1}{r_1} -\frac{1}{r_2} )$
$\frac{1}{f}=(1.5-1) (\frac{1}{20} -\frac{1}{∞} )$
$\frac{1}{f}=(0.5) (0.05 -0)$
$\frac{1}{f}=0.025$
$f=+40\ cm$
Note that it is a converging lens (thicker in the center than the edges) so the focal length must be positive.
