Thursday, November 13, 2014

Subtracting With Regrouping


So, apparently subtraction with regrouping is a big part of third grade. I did not realize this as the lowest grade I had taught before this year was fourth. Fourth graders generally know how to subtract with regrouping, so I have never had to teach it.

This year I was faced with kids who literally had no idea how to subtract with regrouping. These were smart kids - it just hadn't come up yet. 

This lesson was actually one of my first after returning from my very short maternity leave, and, I must say, it was one of my top ten lessons. The majority of my class came in not knowing how to do something and left knowing how to do it. It doesn't get much better than that, does it?

Here's what we did...

I passed out plastic bags filled with hundreds squares, tens sticks, and ones cubes to each student. I then used my own set under the document camera to solve a subtraction problem. First, I made the top number with my pieces, then I tried to take away the bottom number. So, if the top number was 342 I would lay out two ones, four tens, and three hundreds. Then, I would try to take away the bottom number of 158. I would start with ones and say, "How can I possible take away eight ones when I only have two of them?"

Kids figured out pretty quickly that we could break up one of the tens to get enough ones. We repeated this process with the hundreds. I did two examples and then I pointed out that this method, while effective, was super slow. I then shared the "10-second method" otherwise known as the algorithm.

I think the key to this lesson was going back and forth between the blocks and the algorithm. Kids were immediately able to see the connection between moving the blocks around and what we write while using the algorithm. 

This may be a classic third grade teacher lesson, but it was new to me. I hope this post was helpful if you are looking for a way to teach subtraction with regrouping!

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