Thanks for this article. As a father of four, and vehement enemy of all things mathematical (I teach art), seeing this kind of thing coming from my child’s school would have infuriated me. You’ve provided a great explanation for why the teacher marked it incorrect and also why it’s important for a student to understand that. Nicely done. And while you haven’t converted me to a math person, this is a good reminder to me, to trust those people who are experts in their fields to do what they have studied and trained to do.