Subtracting Doubles

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Objective

Students will be able to use addition doubles facts to help them subtract.

Big Idea

The big idea of this lesson is that addition and subtraction facts are related, and every subtraction fact has a related addition fact. Students can think of this inverse relationship when recalling basic subtraction facts.

Activator and Materials

10 minutes

I begin the lesson by having the students turn and talk about what they remember about Doubles Addition Facts.  Replay the Doubles video and have the students sing along.  This should help them remember their doubles facts.  It may be helpful to have the students work as a class to make a list of doubles facts on the board.

Develop the Concept

20 minutes

Students work in partners to choose a doubles fact, and then use their Part Part Whole organizer to build a model, using the two color counters, for this doubles fact.  Then students share with the class which doubles fact they chose and how the model demonstrates the fact.  

Choose one doubles fact and draw the model that matches. Ask the students then to turn and talk about what number sentences can be written based on this model.  The purpose of this question is related to the practice students have had with fact families; that they will be able to correctly identify the subtraction problem that can be written for the model.   

Practice the Concept

20 minutes

Have the students build the problem 14 - 7 using two color counters.  The students have had practice with this and should be able to build the whole (14 in this problem) with red counters.  In order to show that they are subtracting 7, they would flip over the counters (two sided) that they are subtracting.

Have the students turn and talk about what addition problem could represent this model.  If necessary, guide students to think about how they could look at how many yellow counters and how many red counters there are and then how many in all.  Common Core standard 2.OA.2 states that students fluently add and subtract within 20, and know from memory sums of two, one-digit numbers. This lesson gives the students opportunity to practice both addition and subtraction, while recognizing the relationship between addition and subtraction.

Subtracting Doubles Video

Have the students continue to build these number sentences with the two color counters in order to solve all of the Thinking Doubles Practice Page problems.  It is important for the students to build each problem so that they are practicing the strategy.  However, it is important for the students to understand that they may be able to solve the problem using a different strategy.

Student Work Sample

Summarizer

10 minutes

To close, the students share with the entire class their answers for each subtraction problem, including the model that they used and the related doubles fact that they used to help them solve.  

I ask my students an additional question intended to get them to dip more deeply into their thinking, "Think about why they chose the doubles fact you used in your problem."  

The purpose of the summarizer is to have the students explain their thinking.  Many students can think about an answer, but may have difficulty describing their thinking.  Speaking your thinking is also a way to catch your own errors.  Discussions that are built upon sharing are also a critical component in the development of mathematical thinking. These are all important skills to develop early on, as it will help students once the content gets more difficult.  Conversations with your students are also an authentic way to assess your students' understanding of content.