First, you make a factor tree for both 36 and 210 and find the prime factorization of both numbers.
36: 2x2x3x3
210:2x3x5x7
Then, you circle the numbers that appear in both factorizations. (I'll make them bold.)
36: 2x3x2x3
210: 2x3x5x7
Lastly, you multiply the 2 numbers that you circled together (only one pair of the 2 numbers; don't multiply 2x3x2x3, just 2x3).
2x3=6.
The GCF of 36 and 210 is 6.
The GCF is 6.
210 and 420 have a GCF of 210.
Since 210 is a factor of 420, it is automatically the GCF.
The GCF is 6.
The GCF is 70.
The GCF is 42.
The GCF is 15.
The GCF is 14.
The GCF is 105.
The GCF is 6.
The GCF is 1.
The GCF is 14.