Subtracting Fractions with Models

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Objective

The students will be able to subtract fractions using models.

Big Idea

Zero is the Magic Number!

Opener

15 minutes

In this lesson students will be using their knowledge of fraction models to begin subtracting fractions.  They begin by watching a short lesson from Khan Academy and move into an activity that requires them to arrive at zero by subtracting various benchmark fractions.  The lesson ends by students creating their own story problems that require the subtraction of fractions. 

To begin I show students a Khan Academy video titled Subtracting Fractions.  After viewing the video I have the students think about the How Much More Piano Practice Problem.  I show them the initial problem and allow them time with their partner to solve the problem using some sort of fractional model.  After a few minutes I show the students the remainder of the video and then we discuss similarities between their thinking and the thinking presented in the video.  I also ask them to make some general inferences about how subtracting fractions is the same or different from adding fractions(MP 2). 

Practice

30 minutes

To practice subtracting fractions students will do an activity adapted from the Create 3 task in the Georgia State curriculum.  I have included a copy of the task but I decided to make a few modifications so that the task better fits subtracting fractions. 

Instead of having students create 3 using the fractions provided, I ask them to start at 3 and go to zero.  By reversing the process of this task students will be working on subtracting fractions instead of adding them.  During the task the students will be using the number line model to represent the fractions(MP 4).

Each pair of students will need a game board a few counters to keep track of their place on the board.  Students may also use whiteboards and any manipulatives that are available(MP 5).  I copy the game board double sided so that the students may complete a different board if they have additional time. 

At the end of activity we have a class discussion about strategies and difficulties associated with the task. 

Closer

15 minutes

To wrap up today’s lesson I have students create some scenarios in which they would need to subtract a fraction of something to determine the answer.  I allow the students about ten minutes to work with a partner to create, write, and solve two story problems involving subtracting fractions. 

I have some students share their story problems and solutions with the class.  I offer the class an opportunity to respond to the work of their peers and offer feedback(MP 3).