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

final image is formed at infinity if the combined focal length of the two lenses (in contact) becomes 30cm . Thus from lens maker’s formula, $\frac{1}{30}=\frac{1}{20}+\frac{1}{f}$  i. e when another concave lens of focal length 60 mc is kept in contact with the first lens. Similarly, let $\mu$  be the refractive index of a liquid in which focal length of the given lens becomes 30 cm. Then,
  $$\begin{gathered} \frac{1}{20}=(\frac{3}{2}-1)(\frac{1}{R_1}-\frac{1}{R_2}) \\ \frac{1}{30}=(\frac{3/2}{\mu }-1)(\frac{1}{R_1}-\frac{1}{R_2}) \end{gathered}$$
 
From Eqs (i) and (ii),   $\mu =\frac{9}{8}$