Given : $f _{1}=10\, cm ,$
$ f _{2}=20\, cm$
Focal length of plane mirror $f _{3}=\infty$
The resultant power of the combination is
$\frac{1}{ f }=\frac{1}{ f _{1}}-\frac{1}{ f _{2}}+\frac{1}{ f _{3}}$
$\frac{1}{ f }=\frac{1}{10}-\frac{1}{20}+\frac{1}{\infty} $
So $ \frac{1}{ f }=\frac{1}{20} $
$\Rightarrow f =20 \,cm =0.2\, m$
Power of combination
$P =\frac{1}{ f }=\frac{1}{0.2}=5 \,D$
