The greatest number that is a factor of all your chosen numbers:

• find all the factors of each chosen number,

• find any factors that are the same ("common"),

• the largest of those common factors is the Greatest Common Factor.

Abbreviated "GCF". Also called "Highest Common Factor"

Example: the GCF of 12 and 16 is 4, because 1, 2 and 4 are common factors of both 12 and 16, and 4 is the greatest of them.