$($a$)$ Calculate the diffusion coefficient for magnesium in aluminum at $Mg$ in $Al=1\mathrm{.}2{10}^{-4}\frac{m^2}{s}$;$Q_d=130,000\frac{j}{m}ol)($b$)$ What time will be required at $550C$ to produce the same diffusion result $($in terms of concentration at a specific point$)$ as for $15h$ at $450C?$