20: 1, 2, 5, 10, 20 21: 1, 3, 7, 21
1,2,3,4,5,6,10,12,15,20,30,60
273
It is 7
6 numbers can go into 20; they are 1, 2, 4, 5, 10 and 20.
a+21
It is: 21
1 and 7
1 and 3
Any of their multiples
The HCF of 39 and 20 is 1
1, 2, 4, 5, and 10 can go into 20.
The GCF is 7.