1. The problem is to understand the concept of a tracer, which in physics or engineering often refers to a substance or particle used to track the movement or flow within a system. 2. A tracer can be modeled mathematically depending on the context, such as concentration over time or position. 3. For example, if a tracer concentration $C(t)$ decays exponentially, the formula is $$C(t) = C_0 e^{-kt}$$ where $C_0$ is the initial concentration and $k$ is the decay constant. 4. Important rules include understanding exponential decay and how to interpret the parameters. 5. Without a specific mathematical problem, this is a conceptual explanation.