Treat the system as three thin lenses in contact: two plano-concave liquid lenses (one on each side) and one biconvex glass lens.

Using the lensmaker's formula $\frac{1}{f}=(n-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$ with appropriate sign conventions:

Left liquid lens ($n=1.6$): $\frac{1}{f_1}=(1.6-1)\left(\frac{1}{\infty }-\frac{1}{-20}\right)=-\frac{3}{100}$ (after sign analysis).

Glass lens: $\frac{1}{f_2}=(1.5-1)\left(\frac{1}{20}-\frac{1}{-20}\right)=\frac{1}{20}$.

Right liquid lens: $\frac{1}{f_3}=-\frac{3}{100}$.

$\frac{1}{f_{eq}}=-\frac{3}{100}+\frac{5}{100}-\frac{3}{100}=-\frac{1}{100}$

$\therefore f_{eq}=-100$ cm.