To be effective, a certain drug must not drop below 15 mg in the body. A 120 mgs of that drug was administered to a patient. How much time will it take for that drug to reach the 15 mg minimum level. The drug's half-life is 30 minutes.(show how you arrived at your response)

90 Minutes

Explanation:

Original concentration = 120 mg

Half life = 30 minutes

Minimum level = 15 mg

Basically, we are to calculate the time it would take to drop from 120 mg to 15 mg.

The half life of a substance is the time it takes for the original concentration of a substance to drop to half it's starting concentration.

First Half life:

Starting concentration = 120mg

Concentration after half life = 120 / 2 = 60 mg

Second Half life;

Starting concentration = 60 mg

Concentration after half life = 60 / 2 = 30 mg

Third Half life;

Starting concentration = 30 mg

Concentration after half life = 30 / 2 = 15 mg

We have reached the minimum 15 mg level. This means it took three half lives to get there.

Total time taken = No of Half lives * Half life duration = 3 * 30 mins = 90 Minutes