One of my friends shared this picture on Facebook—

(which came via mathtricks.org)—and suggested that the teacher who graded this assignment should not be teaching math at all. I suspected that the grading had fallen prey to a heavy teaching load for an elementary school teacher who might not be as comfortable as they’d like to be with mathematical concepts, so I wrote this response (which I’ve edited slightly):The teacher is doing something rather sophisticated—solving a more general problem—which is what makes it easy to trip up on the apparent simplicity of this question. Consider the following similar questions:

“It took Marie 10 minutes to paint two boards. If she works just as fast, how long will it take her to paint three boards of the same size?”

“It took Marie 20 minutes to saw a board into 5 pieces. If she works just as fast, how long will it take her to saw another board into 6 pieces?”

In the case of the first alternative question I’ve proposed, the teacher’s reasoning would be entirely correct: 10 minutes for 2 boards means 15 minutes for three boards. Although this is not the question that’s being asked, sometimes it’s helpful to think of situations where an incorrect sequence of reasoning becomes correct in order to identify where the mistakes are.

In the case of the second alternative question I’ve proposed, think about how you would solve it. Would you divide the 20 minutes into 5 equal periods of time, or 4? Would you blame someone for dividing by 5 the first time they attempted to solve the problem? Once you figure out that what's important is the 4 cuts it takes, rather than the 5 pieces that are produced, then you can solve any such problem. For example, “If Marie takes an hour to cut a board into 6 pieces, then how long will it take to saw another board into 12 pieces?” (The answer, btw, is

not2 hours.)The reasoning the teacher wrote on the paper is clearly of this latter kind. Their mistake is not in

computation, but in choosing what aspect of the problem deserves attention, namely the cuts in the wood and not the resulting pieces. This leads to nothing more than an “off by 1” error, which is easily corrected. I would be happy to see this reasoning written on a student’s paper, because I would know that only a small correction is needed, after which the student could solve the much more general problem, thanks to a demonstrated understanding of proportion.Math teachers have to be prepared to look for this kind of demonstrated understanding in order to hone in on where a student is making mistakes in their reasoning. This particular case is an example of someone who is teaching math, but probably also a lot of other subjects, and may or may not have training in mathematical thinking. So the more sophisticated concept—proportionality—steps in and overrides a simpler formulation of the problem, which just involves counting. This kind of mix-up is common not just in students, but among all people. Which is why I don't think it’s incompetence, but a symptom of the need for more mathematical training for teachers.

I’m curious how other math teachers would have responded to this discussion. There are certainly those in the math community that can give clearer expression to what I was trying to say. Other commenters on Facebook seemed baffled that a teacher could make this mistake in grading, but I think it’s not such a serious error in reasoning (except that the teacher should have been correcting this on students’ papers, rather than making the mistake on their own).

So, what do you think?