11 doesn't go into any numbers because it is prime, same with 17. 15 only goes into 3 and 5 so that's not it either. 21 only goes into 3 and 7, 26 doesn't go into 4 nor 6. 4 doesn't go into 30 evenly, so that leave just 10 and 12. 3,4,nor 6, go into 10. That leaves only 12 as the final answer.
