Highest common factors and lowest common multiples using prime factors
In some questions the The highest common factor (HCF) of two numbers is the largest number which will divide exactly into both of them, for example, the highest common factor of 24 and 36 is 12. or The smallest positive number that is a multiple of two or more numbers. of two large numbers may need to be found.
It would take a long time to write out all the factors and multiples of 24 and 180 and compare the lists. It would also be easy to make a mistake. A more efficient method is to use prime factors.
Using prime factors to find the HCF and LCM
Numbers can be broken down into prime factors using prime factor trees. When the prime factors of two numbers are known, they can be compared to calculate HCFs and LCMs. This can be a more efficient method than listing the factors and multiples of large numbers.
Example
Find the HCF and LCM of 24 and 180.
Start by writing 24 and 180 as the To multiply. The product of two numbers is the answer to the multiplication of the numbers. The product of 5 and 8 is 40. of their prime factors.
The product of prime factors for 24 are: \(2 \times 2 \times 2 \times 3\)
The product of prime factors for 180 are: \(2 \times 2 \times 3 \times 3 \times 5\)
To find the HCF, find any prime factors that are in common between the products. Each product contains two 2s and one 3, so use these for the HCF.
HCF = \(2 \times 2 \times 3 = 12\)
Cross any numbers used so far from the products.
The product of prime factors for 24 are: \(\cancel2\times\cancel2\times2\times\cancel3\)
The product of prime factors for 180 are: \(\cancel2\times\cancel2\times\cancel3\times3\times5\)
To find the LCM, multiply the HCF by all the numbers in the products that have not yet been used.
LCM = \(12 \times 2 \times 3 \times 5 = 360\)