The two surfaces of a biconvex lens has same radii of curvatures. This lens is made of glass of refractive index $1.5$ and has a focal length $10\;\text{cm}$ in air. The lens is cut into two equal halves along a plane perpendicular to its principal axis to yield two plano-convex lenses. The two pieces are glued such that the convex surfaces touch each other. If this combination lens is immersed in water (refractive index $= 4/3$), its focal length (in cm) is