How many grams of water do you need to add to 600 g of a 15% salt solution to get a new solution containing 10% salt?

Decision.
15% = 0.15; 10% = 0.1%.
600 * 0.15 = 90 (g) salt is contained in each of the solutions.
Suppose you need to add x g of water. Then the mass of the new solution is 600 + x g, and the mass of the salt in it is 10% of the mass of the solution.
We get the equation:
0.1 * (600 + x) = 90; 600 + x = 900;
x = 300.
Answer: 300 g