Answer
\begin{vmatrix}
Isotope & Mass(u) & Abundance\\
^{28}Si & 27.98 & 92.21\% \\
^{29}Si& 29.01 & 4.70\% \\
^{30}Si&29.97 & 3.09\%
\end{vmatrix}
Work Step by Step
1. The sum of all abundances must be equal to 100$\$$ (1):
$$x+0.0470+0.0309 = 1$$
$$x = 1 - 0.0470- 0.0309$$
$$ x = 0.9221 $$
2. Calculate the missing atomic mass:
$$( 0.9221 )( 27.98 \space u)+( 0.0470 )( x )+( 0.0309 )( 29.97 \space u)=28.09 \space u$$
$$ ( 0.0309 )(x) = 28.09 \space u -( 0.9221 )( 27.98 \space u)-( 0.0309 )( 29.97 \space u)$$
$$ x = \frac{28.09\space u - ( 0.9221 )( 27.98 \space u)-( 0.0309 )( 29.97 \space u)}{0.0470 } $$
$$x = 29.01 \space u $$
