After a fire starts in a room, the walls are exposed to combustion products at 950°C. If the interior wall surface is made of oak, how long after exposure to the fire will the wood surface reach its combustion temperature of 400°C? Pertinent data are the following:

h = 30 W/m² · K

T_i (initial) = 21^oC

    For oak:                     ρ = 545 kg/m³

                                       k = 0.17 W/m · K

                                       c_p = 2.385 kJ/kg · K