A slab of stone of area $0.36\,\text{m}^2$ and thickness $0.1\,\text{m}$ is exposed on the lower surface to steam at $100^\circ\text{C}$. A block of ice at $0^\circ\text{C}$ rests on the upper surface of the slab. In one hour $4.8\,\text{kg}$ of ice is melted. The thermal conductivity of slab is (Given latent heat of fusion of ice $= 3.36 \times 10^5\,\text{J kg}^{-1}$)