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A2. HCFs and LCMs : Highest common factor. Vocabulary Because 3 divides into 12 exactly, 3 is called a factor, or divisor, of 12. We also describe 12 as a multiple of 3. Other factors of twelve are 1, 2, 4, 6 and 12. 3 is a factor of 12. 3 is also a factor of 18. Therefore 12 and 18 have a common factor of 3. The other common factors, besides 3, of 12 and 18 are: 1, 2 and 6 6 is the biggest in this list and is therefore called the highest common factor (HCF) of 12 and 18. The highest common factor of 16 and 24 is 8. The highest common factor of 20 and 32 is 4. One way to work out the HCF of two numbers is to write each one as a product of their prime factors, and then pick out all the common factors. 24=2 x 2 x 2 x 3 36=2 x 2 x 3 x 3 The factors common to both are 2 x 2 x 3=12, so 12 is the HCF of 24 and 36. Find the HCF of two numbers yourself and check your answer. The correct answer is shown if you are wrong. Press the button to get started: The HCF=