An ice is encolsed in a cubical container of width W= 200 mm on a side, and the wall of the container is of thickness L= 5 mm and thermal conductivity k= 0.5 W/(m•K). The inner surface of wall is at the fusion temperature of ice (Ti = 0 ⁰C) throughout the process. If the ice is supposed to completely melted within 30 minutes, please estimate the minimum temperature (Ts) of the outer surface of the container to be heated. The density and latent heat of fusion of the ice are  920 kg/m³ and 334 kJ/kg, respectively.