Presently the solution is 8 liters with 75% of it being antifreeze. This means that 75% of the 8 liter (which equals 6 liters) solution is antifreeze. To change the solution, you must add water. In an equation, it looks like this:
6/(8 + x) = 2/5 where x represents the amount of water added & 2/5 represents the amount of the solution that is antifreeze.