The only possibility of heat flow in a thermos flask is through its cork which is 75 cm2 in area and 5 cm thick. Its thermal conductivity is 0.0075 cal/cmsecoC. The outside temperature is 40oC and latent heat of ice is 80 cal g–1. Time taken by 500 g of ice at 0oC in the flask to melt into water at 0oC is