What strikes me about this piece of student work is how clean and predictable their mistake is.

Is this sort of mistake the rule or the exception? Does a mistake like this reflect the fact that many/most student errors are due to coherent mental models, or is it the rarer exception in a world dominated by stormy minds that fling ideas at math less predictably?

Thanks again to Dionn!

I love this mistake. ¬†From their work, what can we infer about what the student knows? Where are they getting confused? How would you help? (Also, if you don’t feel as if you have enough information, feel free to jump in with want information you’d like.)

Elementary teachers? You out there?