Concentration of a Drug in the Bloodstream The concentration (in milligrams per milliliter) of a certain drug in a patient's bloodstream $t$ hr after it has been administered is given by $C(t)=3 t e^{-t / 3} \mathrm{mg} / \mathrm{mL} .$ Find the average concentration of the drug in the patient's bloodstream over the first 12 hr after administration.