Atoxic 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 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?

A. hannah wants to improve her cardiorespiratory fitness levels, so she begins to alternate running on a flat track with running up stadium stairs. this principle is known as a type b tedium c time d frequency 2. charlie is easily winded during soccer practice and wants to improve his cardiorespiratory fitness. which activity would be best for him to choose? a lifting weights b walking from one class to another c running in a 5k race d riding a bike at a leisurely pace 3. felicia just began running this past month, but since she continues to run the exact same route and exact same distance every day, she is growing bored with her runs. this principle is known as a tedium b progression c specificity d reversibility