v/v Concentration is a type of concentration in which we use to determine what would be the percentage of a certain volume of solute (the one with the least volume) by a certain volume of solution (the one with the most volume). The formula for this will be:
%v/v = volume of solute/volume of solution
In this question, we have:
%v/v = 8.7%, which is 0.087
volume of solute = 34.2 mL
Now we add these values into the formula:
0.087 = 34.2/v
v * 0.087 = 34.2
v = 34.2/0.087
v = 393 mL of ethanol solution that contains 34.2 mL of ethanol
