Hello,

Is there a tutor that can tell me how to solve this equation?

A toxic radioactive substance with a density of 4 milligrams per square centimeter is detected in the ventilating ducts of a nuclear processing building that was used 45 years ago. If the half- life of the substance is 20 years, what was the density of the substance when it was deposited 45 years ago?

The density of the radioactive substance when it was deposited 45 years ago was approximately ______ mg divided by cm squared.

(Round to the nearest tenth as needed.)