Here mass of solute (i.e. concentration of F- ion) = 0.2 gm in 500gm toothpaste
so concentration per Kg = 0.2/500 x 1000 gm = 0.4 gm/Kg
Since to find the level of F- ion in ppm, we need to convert the concentration in milligrams as;
concentration of F- ion in mg/Kg = 0.4 x 1000mg /gm = 400 mg/Kg
Hence, level of F- ion in part per million ppm. = weight of solute / weight of solution (in kg)
so, Level of F- ion in ppm = 400 mg/Kg = 400 ppm.