The answer is simple.
First, you must look what is the least among the list. In this case, it is 16x.
Then get the factors of 16x and they are (4,4) , (8,2) and (16,1).
Since you can't then divide it to 36.
Since you can't divide 36 by 16 and 8. You must eliminate them from the list of GCF.
2 and 4 are possible but 4 is the greatest Common Factor which can be divided to both 16 and 36.
