Exploring Subtraction Computation Strategies

During the past few weeks my second grade class has been taking apart and putting together two and three-digit numbers. In the process students have been developing a better understanding of numbers.  They’ve been exposed to using a variety of computation strategies to find the sum and differences of numbers.  Through all of this I’m finding that the students are becoming more confident in their ability to use these different computation strategies more fluently.   Although they’re confident they tend to gravitate towards using one specific strategy for computation.  The traditional algorithm is usually the primary method that they use.  Even though students can add/subtract using that method I found that they weren’t expanding their understanding of other computation strategies.  This was a bit of an issue for me because students started to look at computation as the shortcut and not delve into the understanding of why it works.

After speaking with a few other teachers I decided to use a math task found in this book.  I briefly reviewed the different strategies that we’ve learned this year and gave the students this prompt.

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I wanted to make sure that students showed two different strategies and provided some type of written explanation.  The template I copied also had fields for a number model and explanation boxes.

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The bottom of the sheet was designed for students to be able to check their work using addition.

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I gave the students about 10-15 minutes to complete the formative assessment.  Most of the students tried out the standard subtraction algorithm but had a bit of trouble with the second strategy.  After a few moments students started to dig deep and think of how to take apart numbers using different strategies.  Some of the  students truly had trouble using a different strategy and this was evident in what they produced.  I was impressed with some of the different strategies that students used.

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I wrote feedback on the papers and handed them back to the students the next day. Afterwards, I removed the names off of the papers and shared some of the results with the class.  As a class we decided on the following:

  • Students remembered many of the different computations strategies that were discussed earlier in the year
  • Some of the students invented their own strategies on this particular sheet
  • Students need to strengthen their written explanations
  • Students had some trouble explaining what regrouping means

Next week the class will be setting goals in improving our written responses.  Overall, I feel like this activity helped showcase different computations strategies while bringing awareness to areas the need improvement.  I’d like to use this template with a few other classes later in the year.  Feel free to download and edit this file for your own classroom.

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