Atoxic radioactive substance with a density of 9 milligrams per square centimeter is detected in the ventilating ducts of a nuclear processing building that was used 35 years ago. if the half- life of the substance is 20 years, what was the density of the substance when it was deposited 35 years ago?

After 20 years, the substance would’ve been 18 milligrams. If there had been another 20 years, it would’ve been 36 milligrams, but since it’s only 15 years and 15 is 75% of 20 years, then you’d do .75•18, which is the number you’d have added. This will equal 13.5. Then you do 18+13.5 to get 31.5. This means that 35 years ago, the substance had been 31.5 milligrams.