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A $20$% ethanol solution is mixed with another ethanol solution, say, $S$ 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 $S$ is

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

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