A point object is placed at 30cm from a convex lens (${\mathrm{\mu }}_{\mathrm{g}}=\frac{3}{2}$) of focal length 20cm. The final image of object will be formed at infinity if 

Final image formed at infinity, if the combined focal lengths of the two lenses ( in
contact) becomes 30cm

 $\frac{1}{30}=\frac{1}{20}+\frac{1}{\mathrm{f}}$
$\Rightarrow \mathrm{f}=-60$
i.e when another lens of concave (f=60cm) in contact with first lens.

Similarly$\left(\mathrm{\mu }\right)$ is R.I of liquid, in which focal length of given lens becomes 30cm
$\begin{array}{l}\frac{1}{20}=\left(\frac{3}{2}-1\right)\left(\frac{1}{{\mathrm{R}}_1}-\frac{1}{{\mathrm{R}}_2}\right)\to 1\\ \frac{1}{30}=\left(\frac{\frac{3}{2}}{\mathrm{\mu }}-1\right)\left(\frac{1}{{\mathrm{R}}_1}-\frac{1}{{\mathrm{R}}_2}\right)\to 2\end{array}$
Solving 1&2 we get $\mathrm{\mu }=\frac{9}{8}$