Hcf 18 9 C0A1E4

1. **State the problem:** Find the highest common factor (HCF) of $18$ and $9$. 2. **Formula and rules:** The HCF of two numbers is the greatest number that divides both numbers exactly without leaving a remainder. 3. **Find factors:** - Factors of $18$ are $1, 2, 3, 6, 9, 18$. - Factors of $9$ are $1, 3, 9$. 4. **Common factors:** The common factors of $18$ and $9$ are $1, 3, 9$. 5. **Highest common factor:** The greatest of these common factors is $9$. **Final answer:** The highest common factor (HCF) of $18$ and $9$ is $9$.