1. **State the problem:** Find the Greatest Common Factor (GCF) of 10 and 15. 2. **List the factors:** - Factors of 10 are 1, 2, 5, 10. - Factors of 15 are 1, 3, 5, 15. 3. **Identify common factors:** The common factors of 10 and 15 are 1 and 5. 4. **Find the greatest common factor:** Among the common factors, the greatest is 5. 5. **Conclusion:** Therefore, the GCF of 10 and 15 is $5$.