A greatest common factor is used to compare two or more numbers - a single number cannot have a greatest common factor.
The GCF is 1.
The GCF is 1.
The GCF is 1.
The GCF is 31.
The GCF is 31.
The GCF is 1.
The GCF is 1.
The GCF is: 1
The GCF is 1.
The GCF is 1.
90-59 = 31
The GCF of 21 and 59 is 1.