define the tea in thermosone liter of tea at

Question

Define the Tea in Thermos?

One liter of tea at 90° C is poured keen on a vacuum-insulated container (thermos). The surface area of the thermos walls A=600 cm². The volume among the walls is pumped down to P_o~ 5.10^-6 atm pressure (at room temperature). The emissivity of the thermal and the walls capacity of water C = 4.2.10³ J/kg K. ignore the heat leakage through the stopper, estimate the

a) Net power transfer

b)  Time for the tea to cool from 90°C to 70°C.