A small pond of depth 0.5 m deep is exposed to a cold winter with outside temperature of 263 K . Thermal conductivity of ice is K=2.2 Wm ⁻¹ ~K ⁻¹ , latent heat L=3.4 10^5 Jkg ⁻¹ and density =0.9 10^3 kgm ⁻³ . Take the temperature of the pond to be 273 K . The time taken for the whole pond to freeze is about