How many liters of pure acid must be added to 3 liters of 50% acid solution to obtain a 75% acid solution?

If there are 3 liters of 50% acid solution, there must be 1.5 liters of pure acid and 1.5 liters of another liquid.

As it is, 1.5/x = 1/2, where x = total liters of both liquids (here, 3). We use 1.5 because that's how much we have of the other liquid.

If we set 1.5/x = 1/4, x = 6. This is 3 liters more than our original solution, and we only added pure acid.

Our answer is 3 liters of pure acid.