ExplanationThe chemist now has 10 ounces of acetone in a 30-ounce mixture, so she must have 20 ounces of water.
The question ask how many ounces of acetone must be added to make this mixture a 50% solution. No additional water is added, so the solution must finish with 20 ounces of water.
Therefore, she also needs a total of 20 ounces of acetone, or 10 more ounces than the mixture currently contains.
Note that one trap answer is (B), or 5. This answer is not correct because the final number of ounces in the solution is not 30; when the chemist adds acetone, the amount of total solution also increases— adding 5 ounces acetone would result in a solution that is \(\frac{15}{30+5}\)acetone, which is not equivalent to a 50% mixture.
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