A toxic radioactive substance with a density of 2 milligrams per square centimeter is detected in the ventilating ducts of a nuclear processing building that was used 35 years ago. If thehalf- life of the substance is 20 years, what was the density of the substance when it was deposited 35 yearsago?

The density of the radioactive substance when it was deposited 35 years ago wasapproximately _____mg/cm^2