Concept

When looking at all the factors of two or more numbers, some factors are the same (or common). The largest of these common factors is the greatest common factor (GCF).

Rule

To find the greatest common factor of two numbers:
1. List all the factors of each number. 
2. Find the greatest factor they have in common
This the greatest common factor.

Example

What is the greatest common factor of 42 and 56?

Factors of 42: 1, 2, 3, 6, 7, 14, 21, 42
Factors of 56: 1, 2, 4, 7, 8, 14, 28, 56

Solution

To find the greatest common factor of 42 and 56:

1. Look at all the factors of each number:

Factors of 42: 1, 2, 3, 6, 7, 14, 21, 42
Factors of 56: 1, 2, 4, 7, 8, 14, 28, 56

2. Find the greatest factor they have in common.

Greatest Common Factor: 14

Pre-requisite Skills

Multiplication facts (Grade 3 units 4 & 5)