1. The problem asks for the appropriate letters used to represent the concept of proportion (or probability) in a population and in a sample. 2. In statistics, the population proportion (or probability) is typically denoted by the Greek letter $\pi$ (pi). 3. The sample proportion, which is an estimate of the population proportion based on sample data, is usually denoted by $r$. 4. Therefore, the population letter for proportion is $\pi$ and the sample letter is $r$. This notation helps distinguish between the true proportion in the entire population and the observed proportion in a sample.