Answer
$$^{28}Si \quad 27.98 \quad 92.21 \%$$
$$^{29}Si \quad 28.9 \quad 4.70\%$$
$$^{30}Si \quad 29.97 \quad 3.09\%$$
Work Step by Step
Abundance of $^{28}Si$ = 100%-3.09%-4.70%= 92.21 %
We refer to the periodic table to determine the average atomic mass of Si.
Let m be the atomic mass of $^{29}Si$:
$28.085=0.9221*27.98+0.0470*m+0.0309*29.97$
$m=\frac{28.085-0.9221*27.98-0.0309*29.97}{0.0470}=28.9$
