A thin horizontal disk having a diameter of 15 cm is maintained at a constant surface temperature of 87ºC and buried at a depth of 20 cm in a semi-infinite medium with an adiabatic surface. The thermal conductivity of the medium is 2.7 W/m· ºC and the temperature of the medium a large distance away from the disk (not the adiabatic the surface temperature) is 13ºC. Calculate the heat lost by the disk in watts.