\(\left\{{}\begin{matrix}\%^{24}Mg+\%^{23}Mg=100\\\frac{\%^{24}Mg}{\%^{23}Mg}=\frac{3}{2}\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}\%^{24}Mg=60\\\%^{23}Mg=40\end{matrix}\right.\)
\(\Rightarrow\overline{A}=\frac{60.24+40.25}{100}=24,4\)