Initially it is simply a concave mirror with \(f=-20 \,cm , R=40 \,cm\)

\(\therefore\) Power of mirror \(\left(P_m\right)=\frac{1}{8}\)

\(=\frac{1}{0.2}=5 D\)

Focal length of lens \(=\frac{R}{2(\mu-1)}\)

\(=\frac{40}{2 \times\left(\frac{4}{3}-1\right)}\)

\(=60 \,cm\)

Power of lens \(=\frac{1}{0.5}\)

\(=\frac{100}{50} D\)

\(\therefore\) Equivalent power \(=2 P_L+P_m\)

\(P=\frac{50}{6}\)

Net focal length \(=\frac{1}{P}\)

\(=\frac{6}{50} \times 100\)

\(=12 \,cm\)