Highest Common Factor (HCF)
What is the highest common factor (HCF) of two numbers?
- A common factor of two numbers is a value that both numbers can be divided by, leaving no remainder
- 1 is always a common factor of any two numbers
- Any factor of a common factor will also be a common factor of the original two numbers
- 6 is a common factor of 24 and 30
- Therefore 1, 2 and 3 are also common factors of 24 and 30
- The highest common factor is the largest common factor of the two numbers
- The highest common factor is useful when simplifying fractions or factorising expressions
How do I find the highest common factor (HCF) of two numbers?
- To find common factors:
- write out the factors of each number in a list
- identify the numbers that appear in both lists
- The highest common factor will be the largest factor that appears in both lists
How can I use a Venn diagram to find the highest common factor (HCF) of two numbers?
- Write each number as a product of its prime factors
- 42 = 2×3×7 and 90 = 2×3×3×5
- Find the prime factors that are common to both numbers and put these in the centre of the Venn diagram
- 42 and 90 both have a prime factor of 2
- Put 2 in the centre of the diagram
- Although 3 appears twice in the prime factors of 90, it appears once in the prime factors of 42
- Put a single 3 in the centre of the diagram
- If there are no common prime factors, put a 1 in the centre of the diagram
- 42 and 90 both have a prime factor of 2
- Put the remaining prime factors in the respective regions
- 7 would go in the region for 42
- 3 and 5 would go in the region for 90
- The highest common factor is the product of the numbers in the centre
- The HCF of 42 and 90 is 2×3, which is 6
- If there are no common prime factors then the HCF is 1
How can I use the powers of prime factors to find the highest common factor (HCF) of two numbers?
- Write each number as a product of the powers of its prime factors
- 24 = 23×3 and 60 = 22×3×5
- Find all common prime factors and identify the highest power that appears in both numbers
- The highest power of 2 in both is 22
- 22 is a common factor
- The highest power of 3 in both is 31
- 3 is a common factor
- No other prime number appears in both
- The highest power of 2 in both is 22
- The highest common factor is the product of the common powers of primes
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- The HCF of 24 and 60 is 22×3 which is 12
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Examiner Tip
- The highest common factor of two numbers could be one of the numbers!
- The highest common factor of 4 and 12 is 4
Worked example
Find the highest common factor of 36 and 120.
Write both numbers as a product of prime factors
36 = 2×2×3×3 = 22 × 32
120 = 2×2×2×3×5 = 23 × 3 × 5
Write the prime factors in a Venn diagram
Multiply the common prime factors in the centre
HCF = 2 × 2 × 3
Alternatively, list the factors for each number
36: 1, 2, 3, 4, 6, 9, 12, 18, 36
120: 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, 24, 30, 40, 60, 120
Another alternative is to find the highest common powers of primes
22 and 31 are the highest common powers of primes
HCF = 22 × 31
HCF = 12