A patient was given a 160 mg dose of a drug IV, and 80 mg was eliminated during the first 120 minutes. If the drug follows first-order elimination kinetics, how much of the drug will remain 6 hours after its administration?

One half of the drug dose is eliminated in 120 min, so its elimination half-life = 2 hours. With the passage of each half-life, the amount in the body (or in the blood) will decrease to 50% of a former level. Thus, at 6 hours after drug administration, the amount of drug remaining is 160 divided by (2x2x2) or 160/8=20mg.