$5.$ For a brittle material that has a specific surface energy of $0\mathrm{.}33$ J$/$m$2,$ and flexural strength and elastic modulus values of $88\mathrm{.}1$ MPa and $61$ GPa, respectively, calculate the critical stress $($in MPa$)$ required for the propagation of a surface crack of length $0\mathrm{.}04$ mm$.$