Finding the Greatest Common Factor
(KS2, Year 5)

The Lesson

The greatest common factor of two or more numbers can be found. Imagine we wanted to find the greatest common factor of 12 and 18.

How to Find the Greatest Common Factor

Finding the greatest common factor is easy.

Question

What is the greatest common factor of the numbers 12 and 18?

Step-by-Step:

1

List the factors of 12.
Factors of 12 = 1, 2, 3, 4 ,6, 12
Don't forget: The factors of 12 are the numbers that divide exactly into 12, leaving no remainder.

2

List the factors of 18.
Factors of 18 = 1, 2, 3, 6, 9, 18

3

The largest factor that appears in both lists is the greatest common factor.
Factors of 12 = 1, 2, 3, 4, 6 , 12 Factors of 18 = 1, 3, 6 , 9, 18

Answer:

6 is the greatest common factor of 12 and 18.

Lesson Slides

The slider below gives another example of finding the greatest common factor and an example of why it is useful. Open the slider in a new tab

What Is a Factor?

A factor is a number which divides exactly into another number. The numbers that are multiplied together to make another number are factors.

What Is the Greatest Common Factor?

The greatest common factor is the largest factor that is common to two or more numbers.

Finding the Greatest Common Factor of More Than Two Numbers

In the example in this lesson, the greatest common factor has been been found for two numbers. But the same method applies to any number of numbers.
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See Also

What is the greatest common factor What is a factor? Finding factors