111. Solving a chemistry problem

6. Natural water containing chlorides was diluted 10 times with distilled water. To 15.00 ml of this water was added 10.00 ml of dye and 5.00 ml of distilled water. To equalize the colors of 2.50 ml of a 0.00500 N sodium chloride solution and the test solution, 10.00 ml of the dye and distilled water were added to the sodium chloride solution to 30.00 ml. Determine the concentration of chloride in natural water (in mg/l).