A very thick plate with thermal diffusivity 5.6 x 10-6 m2/s and thermal conductivity 20 W/m ∙ K is initially at a uniform temperature of 325°C. Suddenly, the surface is exposed to a coolant at 15°C for which the convection heat transfer coefficient is 100 W/m2 ∙ K. Using the finite-difference method with a space increment of x = 15 mm and a time increment of 18 s, determine temperatures at the surface and at a depth of 45 mm after 3 min have elapsed.