Subtraction - Equal Change - Stations (2)

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SWBAT explain and use the equal change algorithm to solve 3 digit subtraction problems.

Big Idea

Alternative algorithms open up doors to productive alternative thinking.


1 minutes

I explain to students that they will continue practice the equal change algorithm in several different ways today.  

The materials for the station, the Teacher Station, the Fact Fiesta station, the Student Teachers station and the Opposite Teams station, and information about how to run these stations, are available in Day 1 of this lesson.

Prior to returning to the station rotation, I ask students some questions about patterns/ examples of repeated reasoning that they see when they work on these problems.

Repeated Reasoning

10 minutes

I want the students to be thinking about patterns they find themselves using while they work through these problems.  A recognition of repeated reasoning in this lesson is a thought process that will transfer over to other areas.  Looking for patterns and repeated reasoning helps students develop habits of mind that will later enable them to place knowledge into pre-existing frameworks.

For example, some students will notice that when they are bringing one of the numbers to the closest ten, they are using 10 facts.  (2 + 8, 1 + 9, 3 + 7) and so on.  Others will also notice they repeat this process when they bring one of the numbers to the closest hundred by adding on or subtracting to get there.  (80 + 20, 30 + 60).


50 minutes

Though this is the second day of the stations, I still check on the groups in the first rotation and walk around and supervise the hopefully quick, efficient transition.  Once they are familiar with a given set of centers, it is always slightly more smooth upon being repeated the 2nd day.

The change for today is that at the end of the lesson (or at another time during the day) they are going to need to write up a written explanation of the steps of this process and then use those written steps to try and teach it to someone at home.  It is essential that all the steps are included and also that they can explain the thinking behind the steps.