Try the simplest approach - start with the concentration definition:
Cp = substance mass / solution mass * 100%
You will have to substitute volume for the mass, which is equivalent of assuming specific density = 1 g/ml.
Calculate how much acid and how much solvent is in the 10 litres of starting solution. Add 10 litres of acid - you have both quantity of acid and solvent in the final solution at hand at this moment, so you can use definition once again and lo - you are ready.
Most (if not all) concentration questions can be done this way - you start with definition and end with definition, using your built-in processor for all intermediate steps