A concrete driveway having a thickness of 18 cm attains an essentially uniform temperature of 30C on

Question:

A concrete driveway having a thickness of 18 cm attains an essentially uniform temperature of 30ºC on a warm November day in Texas. A "blue norther" arrives, which suddenly subjects the driveway to a convection boundary with h = 23 W/m2 · ºC and T∞ = 0ºC. How long will it take for the surface temperature of the driveway to drop to 5ºC? Work the problem two ways, using different assumptions.