Greatest Common Factor
A Greatest Common Factor (also seen as GCF or HCF) is the highest factor that is the same for two or more numbers. It is easy to figure out the GCF when you split the two numbers up into their factors. For example, if we are told to find the GCF of 15 and 20, we first split 15 and 20 into their factors.
Factors of 15 are 1, 3, 5, 15
Factors of 20 are 1, 2, 4, 5, 10, 20
The factors that appear in both lists are 1 and 5. The greatest of these is 5, so 5 is the Greatest Common Factor of 15 and 20.