<u>Lets say your GCF is 20 the ones that would NOT apply are</u>
3,6,7,8,9,11,12,13,14,15,16,17,18,19 because those number can NOT equal up to 20 within a multiple
<em>and these are the ones that do apply 1,2,4,5,10,20 because 1*20=20 2*10=20 4*5=20 5*4=20 10*2=20 20*1=20</em>
Hope this helps if you need a better explanation let me know