To find the greatest common factor (GCF) of 15 and 35, we can list the factors of each number:
- Factors of 15: 1, 3, 5, 15
- Factors of 35: 1, 5, 7, 35
The common factors are 1 and 5. The greatest of these is 5.
Therefore, the greatest common factor of 15 and 35 is 5.