Alejandro R. answered 05/24/18
Patient, supportive Tutor for High School/College Math & Science
You can tell that the person who made this question is probably not a chemist! So the total volume of your final mixture is going to be 15 liters + the liters of pure water you are going to add. 15+x.
You want the % of pure water by volume in the end to be 80% (0.8). So you need to set up a fraction
0.8= (volume of pure water in final mixture)/total volume
What makes this tricky is that the 80% acid mixture is presumably 20% pure water. So the volume of pure water in the final mixture is (15*0.2)+x instead of just x.
0.8=(3+x)/(15+x) --> x=45 L