Answer:
6 is the GCF / 6(4+6)
Explanation:
You should know your multiples up to 10, so then do some mental math and think about what number could go in each example:
Try 2: 2,4,6,8,10,12,14,16,18,20,24... except at the end we know there is most likely a greater GCF because two even numbers still remain in the parentheses.
This should be pretty fast since you should realize that three wouldnt work and same with five.
Until you see six
6,12,18,24,30,36