Arithmetic Grade Five

Lesson Eighty-four: Greatest Common Factors

Common factors are factors which two numbers have in common. Consider the following example with the factors of 16 and 20. 

    FACTORS of 20: 1, 2, 4, 5, 10, 20.

    FACTORS of 16: 1, 2, 4, 8, 16.

    Common Factors of 16 and 20: 1, 2, 4.
The common factors of 16 and 20 are 1, 2, and 4.

Here's another example using 30 and 40. 


    FACTORS of 30: 1, 2, 3, 5, 6, 10, 15, 30.

    FACTORS of 40: 1, 2, 4, 5, 8, 10, 20, 40.

    Common Factors of 30 and 40: 1, 2, 5, 10.
The common factors of 30 and 40 are 1, 2, 5, and 10.

To find the common factors you pretty much have to list the factors of each number and then compare the two lists of factors to see which numbers appear on both lists.

In the next practice exercise you will be asked to identify the missing common factor of two numbers.

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