adding in acid

Question

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

Answer 1

In 6 gallons of 50% acid, we have: 3 gallons acid, and 3 gallons water.

Let x denote the pure acid amount to add to the solution to get 70% acid. We have:

(3 + x)/(6 + x) = 70/100, so we have: 100 * (3 + x) = 70 * (6 + x). Solving this equation, we have: x = 4 gallons.