The key here is that your interpretation is problematic. The' maximum number' here refers to the largest of these three numbers or the largest number in 1-9. If it's the former, it's eight, son. That's right. If it is the latter, it is 1, and you are right. The teacher's affirmation is unreasonable, and the questioner is not rigorous. Children can only think of three numbers given in the title.