A 280 mL solution is 20% salt. How much water should be added to make the solution 14% salt?
You need to add 120 mL of water.
In essence, the amount of salt you had in the first solution will be found in the final solution as well.
This means that you can determine how much water to add to the first solution by first calculating how much salt you have
This means that
Let's say that
This means that you can write
So, if you add 120 mL of water to 280 mL of a 20% salt solution, you will get a final solution that is 14% salt.