They are useful in reducing fractions to their simplest forms.
All numbers have factors. Some numbers have some of the same factors as other numbers. These are known as common factors. Write out the lists of factors of the numbers you want to compare. The numbers that appear on both lists are the common factors.
The factors of 7 are 1 and 7. For them to be common, they need to be compared to another set of factors.The factors of 7 are 1 and 7. For them to be common, they need to be compared to another set of factors.
The factors of 13 are 1 and 13. For them to be common, they need to be compared to another set of factors.
You need at least two numbers to find factors in common.
The factors of 12 are 1,2,3,4,6 and 12. For them to be common, they need to be compared to another set of factors.
The factors of 12 are 1,2,3,4,6 and 12. For them to be common, they need to be compared to another set of factors.
The factors of 18 are 1,2,3,6,9 and 18. For them to be common, they need to be compared to another set of factors.
For common factors to exist, there need to be more than one number to compare the factors.
The factors of 67 are 1 and 67. For them to be common, they need to be compared to another set of factors.
The factors of 18 are 1,2,3,6,9 and 18. For them to be common, they need to be compared to another set of factors.
The factors of 17 are 1 and 17. For them to be common, they need to be compared to another set of factors.
The factors of 311 are 1 and 311. For them to be common, they need to be compared to another set of factors.