1. The problem statement, all variables and given/known data When the kidneys eliminate a chemical from the blood, they tend to eliminate a certain proportion each time period. For example, the average person eliminates about 13% of the caffeine in his or her body each hour. In doses larger than 20mg, caffeine can act as a mild stimulant (it can be toxic in large doses). Assume that a 375mL can of Cola has about 45mg of caffeine. Suppose that you have a can of Cola on the hour for three hours. If you had your last Cola at 10 p.m, when can you expect the caffeine that you have ingested to stop having its effect? 2. Relevant equations All the information is supposedly above. The teacher did say that Microsoft Excel could be used. 3. The attempt at a solution I'm really stuck. I asked someone else and they said that the equation they would use would be dC/dt = -.13C. Could someone explain what they meant by this? I don't understand why it's a negative. I don't want to answer, just some assistance as how to start. It's been years since I've used Excel and I really don't know how to integrate the problem. I'm sorry that there isn't much of an attempt at solution, but I'm just stuck as how to start. Thank you.