## Reflection: Developing a Conceptual Understanding Subtraction Houses Part 2 - Section 2: Subtraction Houses Revisited

Watching the students try to figure out how to solve the problem when I had 81 upstairs and 35 downstairs was amazing. They were all sitting around the rug and I built 81 in the top floor and 35 in the bottom floor. Then I said that yesterday we figured out that the bigger number had to be upstairs. Is 81 bigger than 35. They all agreed that it was. So I started to subtract but... "Hey this won't work, there aren't enough guys upstairs to take their friends out from downstairs now what do I do?"

The students took turns trying to switch the numbers (do 5-1 instead of 1-5). So I held up 1 finger on one hand, and 5 on the other. I asked if I could take 5 away from the 1.  One student tried to switch the whole number and put 35 upstairs and 81 downstairs but then we realized that 35 is not the bigger number so we couldn't do that.

We worked at this for about 5 minutes until someone suggested that we take the ten from next door and move it over. We checked to make sure we still had 81 even though we now had 7 tens and 11 ones.

The process took time, but the students were discovering the concept behind subtraction and regrouping. I don't think they will all remember it with just one introduction, but when they tried the process with the blocks, no one tried to switch the numbers. They all went next door and borrowed a ten and traded it for 10 ones.

The concept that you can't just switch the numbers is crucial to double-digit subtraction. The order of numbers matters here. Take the time to really teach this concept before rushing ahead.

Why Can't I?
Developing a Conceptual Understanding: Why Can't I?

# Subtraction Houses Part 2

Unit 11: The Numbers Are Getting Bigger
Lesson 6 of 15

## Big Idea: Who lives in each room of the house and do they have what students need to borrow to complete a double-digit subtraction problem?

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Standards:
Subject(s):
Math, Manipulative Skills, Number Sense and Operations, subtraction, base ten, algorithm, regrouping
35 minutes

### Beth McKenna

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