Two thin lenses of same radius of curvature $R= 15\;cm$ of different nature made of different material one having refractive index $\mu_1 =\large\frac{2}{3}$ and another $\mu_2$ . The lenses are kept in contact and immersed in water of refractive index $\mu_w = \large\frac{3}{4}$. The focal length of the system of lenses in water is $30\;cm$ . What is the value of $\mu_2$

Question

$\begin{array}{1 1} \large\frac{1}{2} \\ \large\frac{2}{3} \\ \large\frac{1}{3} \\ \large\frac{1}{4} \end{array}$