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
- $52\%$
- $50\%$
- $55\%$
- $48\%$