How much salt must be added to 800 grams of a solution that is 15% to obtain a solution that is 20% salt?
You would add 50 g of salt.
We will assume that the density of the solution remains constant (even though the density changes with concentration!).
A 15 % concentration means
So the mass of salt in 800 g of solution is
So you have
You want to add
This also increases the mass of the solution by
You would have to add 50 g of salt.