Black Magic Math

Yesterday I asked my elementary education majors to solve this division problem in as many ways as they could.

A teacher has some crayons. There are 24 crayons in each box. If there are 192 crayons altogether, how many boxes of crayons does the teacher have?

During the sharing out portion, one student said, “I changed it from 192 divided by 24 to 200 divided by 25, because that was easier, and got 8. And that was the right answer. I don’t know why it worked. It’s pretty much black magic.”

Once I had a student who performed black magic on her math all. the. time. She would do something like this: “First I divided 192 by 2 to get 96, and then I divided 96 by 3 to get 32. Then I multiplied the 2 and the 3 and got 6. Then I divided the 24 crayons by 6 and got 4. And then I multiplied that by 2 since I divided by 2 at the beginning, and I got 8, which was the answer. But I have no idea how it worked!”

And while the other students would puzzle over it, I would stare at the board trying to figure out whether it actually did work, because when I looked at it from one angle it looked like a random string of operations, but when I looked at it from another angle it seemed like maybe there was some sense to be made of it all.

I love solutions like these, because sometimes it’s the former, and sometimes it’s the latter. Usually in my classes “black magic math” happens because the student already knows the destination (8 boxes, in this case) and simply performs operations until they land there. That’s what happened in the second solution.* But sometimes behind those operations there turns out to be an underlying mathematical structure that’s completely reasonable. This is true of the first.** The fun is figuring out which it is. The challenge is bringing my students along for the ride without losing them.

What “black magic math” have you encountered, inside the class or out?

* But I think it only takes one change, in the last step, for this black magic solution becomes mathematically justifiable. I’m curious what other readers might think.

** The crayon boxes can help you see the structure in the first solution. 8 boxes of 24 crayons is 192 crayons. If we add a crayon to each box, there are now 8 more crayons (200 altogether) and 25 in each box. The number of boxes remains unchanged. No magic required.

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