Modelling and Problem Solving Question

Hello,

I've been trying to solve this problem for a while, and it's really getting to me. We're supposed to use Microsoft Excel, but I don't know where to start with the problem. It is as follows:

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?

I'm probably making some silly assumptions, but I'm not sure how to start. (Headbang) Thank you.