How much pure acid should be mixed with 6 gallons of a 50% acid solution in order to get a 70% acid solution?

To help with thinking about this, we can say that the 6 gallons of 50% acid contains 3 gallons of pure acid and 3 gallons of filler. This would make the concentration 3/6, or 50%. Adding a gallon of pure acid would shift the concentration to 4/7, or 57.1%.

When the relationship becomes 70%, four gallons of pure acid have been added.

3+4 7
—— = — = 70%
6+4 10

1. 5/82.95% is better than 7/8