How do you work out the lowest common factor and highest common factor between two numbers?

For example if you're doing 10 and 20 the common factors are what numbers are 10 5 2 1 because every single one of those numbers are abled to going into 10 and 20. The Hughes one is 10 and the lowest is 1.
To woke this out you do
1x10=10 1x20=20
2x5=10 2x10=20
But as 3 doesn't go into 10 or 20 you don't count it and even though 4 goes into 20 it doesn't going into 10 and so it can't be a common factor and vice versa.
The next one is 5 and the next one is ten as 5 times 2 is ten and 5 time 4 is 20 then 10 x 1 equals 10 and 10 times 2 equals 20. Hope this helps.
Then you just write which one is the highest and the lowest.

Letter a i would say