Diameter of plano-convex lens is 6 cm and thickness at the centre is 3 mm. If speed of light in material of lens is 2×10⁸m/s, the focal length of the lens is:

The lens is as shown in the figure.
To determine the radius of curvature R of the curved surface, we can write
R² = 3² + (R - 3)²
Solving this, we get
R = 15.15 cm
Speed of light in any medium is c/μ where c = 3 × 10⁸ m/s and μ is the refractive index.
For the lens, c/μ = 2 × 10⁸ m/s
μ = 3 × 10⁸ / 2 × 10⁸ = 1.5
Putting these values in the lens makers formula for plano convex lens, we have,
1/f = (μ - 1)(1/R) = (1.5 - 1)(1/15.15) cm⁻¹
Solving this, we get
f ≈ 30 cm