In the figure shown here, what is the equivalent focal length of the combination of lenses (Assume that all layers are thin)?

This will be a combination of 3 lenses placed in air
$\begin{array}{l}\frac{1}{f_1}=(1.6-1)\left[\frac{1}{\mathrm{\infty }}-\frac{1}{20}\right]=\frac{-0.6}{20}\mathrm{cm}\\ \frac{1}{f_2}=(1.5-1)\left[\frac{1}{20}-\left(\frac{1}{-20}\right)\right]=\frac{0.5}{10}\mathrm{cm}\\ \frac{1}{f_3}=(1.6-1)\left[\frac{1}{(-20)}-\frac{1}{\mathrm{\infty }}\right]=\frac{-0.6}{20}\mathrm{cm}\end{array}$
The combined focal length is given by
$\begin{array}{l}\frac{1}{f_{\text{eq }}}=\frac{1}{f_1}+\frac{1}{f_2}+\frac{1}{f_3}=\frac{-0.6}{20}+\frac{0.5}{10}+\frac{-0.6}{20}\\ \frac{1}{f_{\text{eq }}}=\frac{-0.1}{10}\Rightarrow f_{\text{eq }}=-100\mathrm{cm}\end{array}$