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Mathematics, 28.07.2019 05:10 SKYBLUE1015

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 55 years ago. if the half- life of the substance is 20 years, what was the density of the substance when it was deposited 55 years ago?

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