10 -cm-thick large food slabs tightly wrapped by thin paper are to be cooled in a refrigeration room maintained at $0^{\circ} \mathrm{C}$. The heat transfer coefficient on the box surfaces is $25 \mathrm{~W} / \mathrm{m}^2 \cdot{ }^{\circ} \mathrm{C}$ and the boxes are to be kept in the refrigeration room for a period of 6 h . If the initial temperature of the boxes is $30^{\circ} \mathrm{C}$ determine the center temperature of the boxes if the boxes contain (a) margarine ( $k=0.233 \mathrm{~W} / \mathrm{m} \cdot{ }^{\circ} \mathrm{C}$ and $\left.\alpha=0.11 \times 10^{-6} \mathrm{~m}^2 / \mathrm{s}\right),(b)$ white cake $\left(k=0.082 \mathrm{~W} / \mathrm{m} \cdot{ }^{\circ} \mathrm{C}\right.$ and $\alpha=0.10 \times 10^{-6} \mathrm{~m}^2 / \mathrm{s}$ ), and (c) chocolate cake ( $k=$ $0.106 \mathrm{~W} / \mathrm{m} \cdot{ }^{\circ} \mathrm{C}$ and $\left.\alpha=0.12 \times 10^{-6} \mathrm{~m}^2 / \mathrm{s}\right)$.