The problem says that there are 3 equal parts (volumes) of water and 2 equal parts (volumes) of acid. It means 5 parts of the mixture liquid, in all. 5 parts comprise 12.5 liters - hence, each part is= 2.5 liters. Thus you get the answer: 3*2.5 liters = 7.5 liters of water and 2*2.5 liters = 5 liters of the acid.