Answer:
The answer is: 25 %
Step-by-step explanation:
In order to know the concentration, we need to know the amount of solution we have, that is: 25 ml of acetone + 75 ml of water = 100 ml of solution. Now, as the units are in volume, and we need to know te concentration in %v/v (percentage volume/volume), we can operate normally (in the same units). The concentration in % v/v is going to to be the amount of the solute over the amount of solution multiplied by 100, and in this case the solute is the acetone:
%v/v = (25/100) * 100 = 25 %