Steel is sequentially heated and cooled (annealed) to relieve stresses and to make it less brittle. Consider a 100-mm-thick plate (k = 45 W/m ∙ K. p = 7800 kg/m³ ∙ c_p = 500J/kg ∙ K) that is initially at a uniform temperature of 300°C and is heated (on both sides) in a gas-fired furnace for which T_∞ = 700°C and h = 500 W/m² ∙ K. How long will it take for a minimum temperature of 550°C to be reached in the plate?