Question

what are the common factors of 15,45 and 90

Answer 1

Answer= 1,3,5,15

which are in essence all on the factors of 15

Answer 2

5,3 and 15 
because the factors of 15:5,3,15,1
45:5,9,15,3 
90:3,30,15,6,9,10,90 
and etc