A sample is taken that has 400,000,000 particles in it. If 20 of these particles are a dangerous chemical, what is the concentration of this chemical in parts-per-million (ppm)?

Answer 1

20 parts in 400 million would be 10 parts in 200 million or 1 part in 20 million or 1/20 part in 1 million or 0.05 ppm.

(20*1,000,000/400,000,000) = 0.05 ppm