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A $20\%$ ethanol solution is mixed with another ethanol solution, say, $\text{S}$ of unknown concentration in the proportion $1:3$ by volume. This mixture is then mixed with an equal volume of $20\%$ ethanol solution. If the resultant mixture is a $31.25\%$ ethanol solution, then the unknown concentration of $\text{S}$ is

1. $52\%$
2. $50\%$
3. $55\%$
4. $48\%$

1
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