A thin equiconvex lens of refractive index $\large\frac{3}{2}$ and radius of curvature 30 m is put in water $(RI= \large\frac{4}{3})$. Its focal length is

Question

$(a)\;0.15\;m \\ (b)\;0.45\;m \\ (c)\;0.3\;m \\ (d)\;1.2\;m $