To find the highest common factor (HCF) of 66 and 110 using their prime factors:

1. Prime factorization of 66:

   • $66 = 2 \times 3 \times 11$

2. Prime factorization of 110:

   • $110 = 2 \times 5 \times 11$

3. Identify common prime factors:

   • Both numbers have the prime factors $2$ and $11$ in common.

4. Multiply the common factors to find the HCF:

   • $2 \times 11 = 22$

Therefore, the highest common factor (HCF) of 66 and 110 is 22.

Would you like a more detailed explanation, or do you have any questions?

Here are some related questions that might expand your understanding:

1. What is the difference between HCF and LCM (Least Common Multiple)?
2. How can we use the prime factorization method to find the HCF of three or more numbers?
3. Why is prime factorization helpful in finding the HCF?
4. Can the HCF of two numbers ever be greater than the smallest number?
5. How would we find the HCF of numbers with no common prime factors?

Tip: When working with HCF and prime factorization, list all prime factors carefully to avoid missing common factors.