Let the volume of 20% ethanol solution in first mixtures = x so
Volume of S = 3x
Total volume of first solution = 4x
Total volume of final solution =2×4x=8x
Volume of ethanol solution in final mixture = x + 4x = 5x ,
So volume of S in final solution = 3x
Thus (31.25-20)/(s-31.25) = 3x/5x = 3/5
56.25=3s-93.75
3s=150
s=50%