15%, the mass of the concentration was added to 600 g of a saline solution of water, concentration

15%, the mass of the concentration was added to 600 g of a saline solution of water, concentration of 50% 240 g of a saline solution of water.

First action: 600 * 15/100 = 90 (g) – salt content in water weighing 600 g;
Second action: 240 * 50/100 = 120 (g) – salt content in water weighing 240 g;
Third action: 240 = 600 = 840 (g) – the total mass of the mortar mixture;
Fourth action: 120 + 90 = 210 (g) – salt content in the total mass of the mortar mixture;
Sixth action: 0.25 * 100 = 25% – the percentage of salt in the total mass of the mortar mixture.
Answer: 25%.