Em tham khảo nha :
\(\begin{array}{l}
{M_{tb}} = \dfrac{{486,54}}{{20}} = 24,327dvC\\
{\% ^x}Mg = 100 - 78,6 - 11,3 = 10,1\% \\
{M_{tb}} = \dfrac{{78,6 \times 24 + 11,3 \times 26 + 10,1 \times x}}{{100}} = 24,327\\
\Rightarrow x = \dfrac{{2432,7 - 78,6 \times 24 - 11,3 \times 26}}{{10,1}} = 25\\
^{25}Mg
\end{array}\)