Unknown concentration in mixture

Let concentration of S be $x\%$. First mixture combines 1 part of 20% solution and 3 parts of S ($x\%$). Concentration of first mixture $= \frac{1(20) + 3(x)}{4} = \frac{20 + 3x}{4}\%$.

This mixture is then mixed with an equal volume of 20% ethanol solution. So resultant concentration is the average of $\frac{20+3x}{4}\%$ and $20\%$, which equals $31.25\%$: $\frac{\frac{20 + 3x}{4} + 20}{2} = 31.25$ $\frac{20 + 3x}{4} + 20 = 62.5$ $\frac{20 + 3x}{4} = 42.5$ $20 + 3x = 170 \implies 3x = 150 \implies x = 50\%$