2023 H2 Chem P1 Q8 — Atomic structure Solution

The answer is C (5.54 %).
Let x be the percentage of \(^{29}\text{Si}\). The percentages add up to 100, so the percentage of \(^{30}\text{Si}\) is \(100 - 92.23 - x = 7.77 - x\).
\(\frac{92.23}{100}(28) + \frac{x}{100}(29) + \frac{7.77 - x}{100}(30) = 28.10\)
\(25.8244 + \frac{233.1 - x}{100} = 28.10\)
\(28.1554 - 0.01x = 28.10\)
\(x = 5.54\)
So \(^{29}\text{Si}\) is 5.54 % (and \(^{30}\text{Si}\) is \(7.77 - 5.54 = 2.23\) %).
A: 2.23 % is the percentage of \(^{30}\text{Si}\), not \(^{29}\text{Si}\).
B: 3.89 % comes from splitting the remaining 7.77 % equally between the two isotopes, which ignores the \(A_r\) of 28.10.
D: 7.77 % is the combined percentage of \(^{29}\text{Si}\) and \(^{30}\text{Si}\).