To work this out, you need to write both numbers as their prime factors:
20 = 2x2x5
12 = 2x2x3
The next step is to identify any common prime factors. In this case both numbers have two 2s as prime factors. Multiply these together and you get 4. Therefore 4 is the HCF of 20 and 12.
the hcf of 12 and 20 = 4
4
4
The GCF is 12.
The GCF is 4.
The GCF is 2.
The GCF of 12 and 20 is 4.
The GCF is 4.
The GCF is 2.
20/12 = 5/3 by dividing the numerator and denominator by their HCF which is 4
The HCF of 12 and 60 is 12
The GCF is 12.