A copper rod with a diameter of 3 cm and a height of 5 cm is initially at a uniform temperature of 100°C. It is now exposed to an air stream at 20°C with a heat transfer coefficient of 200 W/m²°C. How long would it take for the copper road to cool to an average temperature of 28°C? (p= 8950 kg/m³ k= 400 W/m °C,)