Calculate how many grams of water must be added to 300 g of a 20% aqueous salt solution to make a 12% solution.
When additional water is added to an aqueous solution, the concentration of that solution decreases. This is because the number of moles of the solute does not change, while the volume of the solution increases. We can set up an equality between the moles of the solute before the dilution (1) and the moles of the solute after the dilution (2). Since the moles of solute in a solution is equal to the molarity multiplied by the liters, we can set those equal.
The Dilution lesson contains more information and solved examples.