You may have seen all over Facebook lately a meme about the allegedly terrible Common Core math standards that evil President Obama is imposing on innocent school children that supposedly teaches them a completely convoluted way of doing addition and subtraction. It looks like this:
The right wing is throwing a major league tantrum over this. Hemant Mehta, who actually teaches math, debunks the whole thing.
On the surface, it seems ridiculous. The top makes sense; the bottom is silly; screw you, Common Core!
Except that the top doesn’t make sense, the bottom does, and the connection to Common Core is completely misunderstood. (Says this math teacher.)
Here’s what’s going on: The top is how most of us learned subtraction. I’m sure your teachers taught you what was going on mathematically, but do you really remember what they said? Probably not. For us, it’s just an algorithm. You can do it without thinking. You hope there’s no “borrowing” of numbers involved, but if you had to do it by hand, you could probably pull it off.
The problem with that method is that if I ask students to explain why it works, they’d have a really hard time explaining it to me. They might be able to do the computation, but they don’t get the math behind it. For some people, that’s fine. For math teachers, that’s a problem because it means a lot of students won’t be able to grasp other math concepts in the future because they never really developed “number sense.”…I know. That’s still ridiculous. Well, consider this: Suppose you buy coffee and it costs $4.30 but all you have is a $20 bill. How much change should the barista give you back? (Assume for a second the register is broken.)…
Instead, you’d just figure it out this way: It’d take 70 cents to get to $5… and another $15 to get to $20… so you should get back $15.70.
That’s it. That’s the sort of math most of us do on a regular basis and it’s exactly the sort of thinking the “new” way in the picture is attempting to explain. Granted that was an *awful* example to use, but that’s the idea. If students can get a handle on thinking this way instead of just plugging numbers into a formula, the thinking goes, it’ll make other math skills much easier to understand.
He also points out that this is not mandated by Common Core at all, it’s just the way some states and local school districts are implementing the standards. But those are facts, so they won’t matter one bit to the right wing.