There cannot be a greatest common factor (GCF) of just one number. To be common there need to be at least two numbers. If you find all the factors of two or more numbers, and you find some factors are the same ("common"), then the largest of those common factors is the Greatest Common Factor.
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The greatest common factor of 21 and 99 is 3. 21's factors are 1,3,and 7 99's factors are 1,3,9,11 33 (there may be more factors of 99). The highest factor that both number have in common are 3.
The Highest Common Factor (HCF) of 99 is 99. This is because 99 is a prime number, meaning it only has two factors: 1 and 99. Since there are no other common factors between 99 and any other number, the HCF remains 99.
Highest common factor is 7.
The highest common factor is: 20
The greatest common factor of 55 and 99 is 11.The GCF of 55 and 99 is 11.