25% of the solution is acid so there is 25 ounces of acid. I can set up a proportion 25/x = 0.05(remember that percentages must be divided by 100) where x is the amount of water that dilutes 25 ounces of acid into a 5% solution. Multiply both sides by x to get 25 = 0.05x. Now multiply by 20 to get x by itself. x is equal to 500. The question asks how much water should be added so 500 - 100. 400 oz of water should be added to dilute it to a 5% acid solution.