At the end of the day, the sergeant has 400 offender files that have been processed and 100 that still need to be processed. What percentage of the 500 files have not been processed?

Solution:
1. Total files: 500
2. Files not processed: 100

3. Calculate the percentage of files not processed:
- Percentage formula: \text{Percentage} = \left(\frac{\text{Part}}{\text{Whole}}\right) \times 100
- Substitute the known values:
\text{Percentage} = \left(\frac{100}{500}\right) \times 100

4. Simplify:
\text{Percentage} = 0.2 \times 100
\text{Percentage} = 20

5. Therefore, the percentage of files that have not been processed is 20\%.