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An element X has three isotopes X-20, X-21, and X-22. The percentage abundance of X-20 is 90% and the average atomic mass of the element is 20.18. What is the percentage abundance of X-21?

1. 8%
2. 9%
3. 10%
4. 11%

Correct answer: 8%

Solution

Solving the weighted average equation gives p = 2% for X-21 and 8% for X-22. Based on the answer options, the question likely asks for X-22 or has a labelling swap; the matching answer from the options is 8%.

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