Francisco EDCP342 Blog
Monday, September 21, 2026
Eisner’s Curriculum
Wednesday, September 16, 2026
The Good, the Bad and the Ugly
Tuesday, September 15, 2026
The Locker Problem
When I first read the blog post presenting the problem, I immediately concluded that each student changes the state of the lockers that have a number that is a multiple of the student’s number. That didn’t help much. I had to think of it from the lockers’ point of view.
Yeah, that’s not that hard. The locker will be changed by the students whose numbers are a factor of the locker’s number. Okay. If the lockers start open, lockers with an even number of factors would stay open and lockers with an odd number of factors would be closed. Easy. Final answer. Done.
Then I started thinking if there was a better answer for that. But how do I know which numbers have an even or odd number of factors? And I thought I’d have to brute force my way through it. No, it can’t be! That’s not how we do things. So I started thinking about factors, and as it usually goes I thought that prime numbers have only 1 and themselves as factors! That’s it. Prime numbers have an even number of factors, everything else must have an odd number, right? Then just to check I thought of 6. And it turns out that 6 has 1, 2, 3, and 6 as factors. Four factors. Four is even… Wait. Of course! Factors come in pairs. Always! The factors of a number n always come as a×b = n. But then does that mean every single number has an even number of factors and therefore every door is open? Can’t be!
I had to try and brute force the start. Starting with 1 I found that it had only itself as a factor. Wait a second there. 1 is a weird number, so maybe only 1 is closed and everything else is open? Let’s keep checking before we conclude that… Continue brute forcing: 2 has two factors, 3 has two factors, 4 has... Three? Wait! How? What about the factor pairs? 4 is 1×4 and 2×2. OOOH! One of the pairs is a number times itself! So that is a pair, but that number only shows up once in the list of factors. That is the odd factor! The factor that doesn't have a pair other than itself! The square root of the number! It has to be it! Perfect square numbers have an odd number of factors. Also, 1 is a special number in many ways, but also it is a perfect square. Of itself! Brute forced my way to 10 just to check and, as expected, locker 9 was closed!
That’s a beautiful problem. That is a problem that would make me enthusiastically tell the students “that’s why math is the most beautiful thing in the universe!” To which I know most students would just sigh, but such is life…
I built a small interactive visualisation here to help see the first 50 lockers and the first 50 students (we don't need more students because no student after the 50th would touch the first 50 lockers).
Interactive Visualisation
Monday, September 14, 2026
Instrumentality, relationality, and terrible memory.
I've always had a problem with pure instrumental understanding. I imagine part of that is because I have terrible memory, but also because I always wanted to know why. I was curious and loved the feeling of finding things out. I understand some people just want to know the easiest way to solve something, a "just give me the formula" kind of approach. Shortcuts, mnemonics, and memorizing formulas can all help with efficiency, but for me that never really stuck. Having a really bad memory could make me remember the formulas wrong. Memorizing the whole formula and mistaking a plus with a minus sign could sound like a small mistake, but that could also be as big as mistaking the gravity going down with it going up. I've seen a lot of sign mistakes on memorized formulas or on calculations but never found someone who thought gravity was pushing things up. That just shows how fragile the instrumental understanding can be, with small mistakes being huge disconnects with reality. Not that focusing on relational understanding makes calculations fail proof, but at least, if you know what the big picture is, you can look at the result and say "this doesn't seem right", and double-check your work. If you just follow a procedure with no understanding of the relations between that and the world around you, the result is just a number, and a wrong number and a right number are both just numbers.
I agree with most of what was said, including the part where the author mentions the possible reasons for instrumental understanding. But I think when the author mentioned the difficulty of assessing relational understanding, the biggest mistake was trying to do that with traditional math questions. "Solve this" and "calculate this" questions will only check the execution, not necessarily the thinking. Maybe assessments should just be different. Make some math assessments be more like humanities studies, asking students things like "what would be the next step?", or "why would we do this?", or "when would we use this?" instead of just asking for final answers. Math has number problems and word problems, but those are usually just answered with numbers. Maybe math also deserves word problems with word answers.