Question

what are the common factors of 12 and 15

Answer 1

12 = 2*2*3
15 = 3*5

common factor = 3

Answer 2

write down the factors of both, and see what numbers the two have in common. I'll start with the easiest

12
1 * 12

15
1 * 15

Answer 3

and keep going down.

Answer 4

oh yes - i forgot 1

Answer 5

common factors are 1and 3