We can find the greatest common factor of 20 and 30 by finding the factors of the numbers.

Factors of 20 are 1 , 2 , 4 , 5 , 10 , 20

Factors of 30 are 1 , 2 , 3 , 5 , 6 , 10 , 15 , 30

Here, The common factors of 20 and 30 are 1 , 2 , 5 , 10

The greatest number among 1 , 2 , 5 , 10 is 10

Therefore,

The greatest common  factor of 20 and 30 is 10