A large plastic casting with thermal diffusivity 6.0 x10^7 m2 /s is removed from its mold at a uniform temperature of 150 degrees C. The casting is then exposed to a high-velocity air stream such that the surface experiences a sudden change in temperature to 20 degrees C. Assuming the casting approximates a semi-infinite medium and using a finite-difference method with a space increment of 6mm, estimate the temperature at a distance of 18 mm from the surface after 3 min have elapsed.