Diameter of a plano - convex lens is $6\;cm$ and thickness at the centre is $3\;mm$. If speed of light in material of lens is $2 × 10^8\; m/s,$ the focal length of the lens is :

Question

$\begin{array}{1 1} (1)\;15\;cm \\ (2)\;20\;cm \\ (3)\;30\;cm \\ (4)\;10\;cm \end {array}$