The lens is as shown in the figure.To determine the radius of curvature \\\\( R \\\\) of the curved surface, we can write\\\\( R^2=3^2+(R-\\cfrac{3}{10})^2 \\\\)Solving this, we get\\\\( R=15.15 cm \\\\)Speed of light in any medium is \\\\(\\dfrac{c}{\\mu} \\\\) where \\\\( c=3\ imes 10^8 m/s \\\\) and \\\\( \\mu \\\\) is the refractive index.For the lens, \\\\( \\dfrac{c}{\\mu}=2\ imes10^8 m/s \\\\)\\\\( \\mu=\\dfrac{3\ imes10^8}{2\ imes10^8}=1.5 \\\\)Putting these values in the lens makers formula for plano convex lens, we have,\\\\(\\dfrac{1}{f}=(\\mu-1)(\\dfrac{1}{R})=(1.5-1)(\\dfrac{1}{15.15}) cm^{-1} \\\\)Solving this, we get \\\\( f\\approx 30 cm \\\\)