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−310)2
Solving this, we get
R=15.15cm
Speed of light in any medium is cμ where c=3×108m/s and μ is the refractive index.
For the lens, cμ=2×108m/s
μ=3×1082×108=1.5
Putting these values in the lens makers formula for plano convex lens, we have,
1f=(μ−1)(1R)=(1.5−1)(115.15)cm−1
Solving this, we get f≈30cm