(b) PPM or Parts Per Million is expressed as 1 milligram of substance per kilogram of parent substance. Since, mass of toothpaste =500g=0.5kg, and mass of fluoride ion =0.2g=200mg,0.5kg toothpaste contains 200mg fluoride ions, 1kg toothpaste will contain 200×2=400mg fluoride ions. Hence, concentration of fluoride ion is 400ppm or 4×102ppm.