What is the technique to get the GCF?

Answer 1

-- List all the factors of the first number.
-- List all the factors of the second number.
-- If there are more than two numbers, then list all the factors of each of the rest of them.
-- Inspect the lists, to see what factors show up on every list.
-- Pick the greatest factor that shows up on every list.

Answer 2

Example: 30 and 42

Factor them.

2 x 3 x 5 = 30

2 x 3 x 7 = 42

Select the common factors.

2 x 3 = 6, the GCF