\(\left\{{}\begin{matrix}\%Mg=\dfrac{24.1}{120}.100\%=20\%\\\%S=\dfrac{32.1}{120}.100\%=26,667\%\\\%O=\dfrac{16.4}{120}.100\%=53,333\%\end{matrix}\right.\)
\(\left\{{}\begin{matrix}\%Al=\dfrac{27.1}{213}.100\%=12,676\%\\\%N=\dfrac{14.3}{213}.100\%=19,718\%\\\%O=\dfrac{16.9}{213}.100\%=67,606\%\end{matrix}\right.\)
\(a.\%Mg=\dfrac{24\times100\%}{120}\)\(=20\%\)
\(\%S=\dfrac{32\times100\%}{120}\)\(=26,66\%\)\(\%\)
\(\%O=100\%-\left(20\%+26,66\%\right)\)\(=53,34\%\)
\(b.\%Al=\dfrac{27}{213}\)\(=12,67\%\)
\(\%N=\dfrac{42}{213}\)\(=19,71\%\)
\(\%O=100\%-12,67\%-19,71\%=67,62\%\)