You’ll want to find the biggest number that you can multiply something by and get either of the numbers in the question. So for example in question #1 - which of the three answers can you multiply by something and get both 4 AND 10 ? Well, you can multiply 1 x4=4 and 1x10=10 so one IS a common factor but is it the biggest? You can’t multiply anything bu 5 to get 4 (obviously) so that’s out. What about 2? You can multiply 2x2=4 and 2x5=10 so 2 is also a common factor. Between 1 and 2 - 2 is the GREATEST common factor (GCF)
