The thickness at the centre of a plano- convex lens is \(3 \mathrm{~mm}\) and the diameter is \(6 \mathrm{~cm}\). If the…

The thickness at the centre of a plano- convex lens is \(3 \mathrm{~mm}\) and the diameter is \(6 \mathrm{~cm}\). If the speed of light in the material of the lens is \(2 \times 10^8 \mathrm{~ms}^{-1}\). The focal length of the lens is:
[JEE Main 2021, 17 Mar (Shift 1)]