A sphere of radius 5 mm is initially in equilibrium at 400℃ in a furnace. It is suddenly removed from the furnace and dipped in a well-stirred water bath at 20℃, with a convection heat transfer coefficient of 500 W/(m²K). For the given range of temperatures, the thermophysical properties of the material of the sphere are: density ρ = 3000 kg/m³, thermal conductivity k = 10 W/(mK), and specific heat c = 1000 J/(kgK). Neglecting radiation heat transfer, the time required for the centre of the sphere to cool from 400℃ to 50℃ is ________ seconds (rounded off to 2 decimal places).