Answer: 28.1 amu
Step-by-step explanation:
Mass of isotope 1 = 27.98 amu
% abundance of isotope 1 = 92.21% =

Mass of isotope 2 = 28.98 amu
% abundance of isotope 2 = 4.70% =

Mass of isotope 3 = 29.97 amu
% abundance of isotope 2 = 3.09% =

Formula used for average atomic mass of an element :

![A=\sum[(27.98 )* 0.922+(28.98)* 0.047+(29.97)* 0.0309]](https://img.qammunity.org/2020/formulas/chemistry/high-school/1g0b5l7abhn3r91qftbwjon3mngz4ponyx.png)

Therefore, the average atomic mass of silicon is 28.1 amu