A large slab has properties of common building brick and is heated to a uniform temperature of

Question:

A large slab has properties of common building brick and is heated to a uniform temperature of 40ºC. The surface is suddenly exposed to a convection environment at 2ºC with h = 25 W/m2 · ºC. Calculate the time for the temperature to reach 20ºC at a depth of 8 cm.