Teachers have always told me that paper-and-pencil subtraction when problems call for regrouping is hard to teach and hard for students to learn. Much harder than addition. So why subtract when you can always add? That’s what my friend Nicholas thought, and he taught me how.
When a workshop participant raised his hand to offer how he was taught to subtract when he was in elementary school, I was surprised by the process he described. He called it the “trouble-coming method” of subtraction. I hadn’t ever seen it before, and I’ve never seen it since.
Here’s another post about subtraction. I’ve long been a fan of having students invent and use their own methods for computing, as long as they can explain why their strategies make sense. Here’s what I learned from Jesús, a fifth grader.
This post is about subtraction, which is typically difficult for students to learn and for teachers to teach. Think about 503 – 398, for example. To estimate the answer, I can change the problem to 500 – 400 (rounding 503 to 500 and 398 to 400). That gives me an estimate of 100, which I know is close. But how can I know if the actual answer to 503 – 398 is greater or less than 100? I raised this question with third graders.