The concentration of a drug in the body decreases exponentially
after a dosage is given. In one clinical study, adult subjects
averaged 12 micrograms/milliliter (mcg/mL) of the drug in their
blood plasma 1 hr after a 1000-mg dosage and 5
micrograms/milliliter 5 hr after dosage. a) Find the value k,
and write an equation for an exponential function that can be used
to predict the concentration of the drug, in
micrograms/milliliter, t hours after a 1000-mg dosage. b)
Estimate the concentration of the drug 2 hr after a 1000-mg
dosage. c) To relieve a fever, the concentration of the drug
should go no lower than 3 mcg/mL. After how many hours will a
1000-mg dosage drop to that level?