Em tham khảo nha:
\(\begin{array}{l}
3)\\
a)\\
\overline {{M_B}} = 10 \times 18,89\% + 11 \times 81,11\% = 10,8111\\
b)\\
\overline {{M_{{\rm{Ar}}}}} = 36 \times 0,34\% + 38 \times 0,06\% + 40 \times 99,6\% = 39,9852\\
4)\\
{\% ^{80}}Br = 100 - 54,5 = 45,5\% \\
\overline {{M_{Br}}} = 79 \times 54,5\% + 80 \times 45,5\% = 79,455
\end{array}\)