3. A plastic sphere of 5mm in size and originally at 80 degC is cooled in a bed where the air temperature can be assumed to be constant at 30 degc. The density of the solid is 1,100 kg/m3 and the thermal conductivity 0.13 W/m.K and the specific heat capacity 1,700 J/kg.K. The external heat transfer coefficient is 50 W/ m2.K. How long does it take for the average solid temperature to reach 35 deg C? *