List the factors of each number. Compare the lists to find the greatest number common to both. This is the HCF. Draw factor trees close for each number. Write each number as a product of its prime ...

For example, the common factors of 40 and 16 are 2, 4 and 8. The HCF of 40 and 16 is therefore 8. This is the largest number that 'goes into' both numbers. Find the HCF of 12 and 20. The factors ...