Diameter of a plano-convex lens is 6 cm and thickness at the centre is 3 mm.

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

[JEE-Mains- 2013]