Lesson: Subtraction Modeled as Comparing

Objective: Students will use island models on the geoboard to find the difference of two fractions by finding “how much larger” one number is.

Materials:

• A set of fraction islands and a geoboard shared with a partner

• Fraction Island sheet

• Pencil

• Colored Pencils

Investigation:

1. Make two 3X4 units on the geoboard, then model 1/2 in one unit and 1/3 in the other unit.

   1. Move the pink island to the other unit and place it on the brown island.

   2. Compare the two islands and describe how much larger the brown one is.

   3. Can you use another island to cover the part of the brown island that is “sticking out”?

   4. What part of the unit does this island model?

   5. Click here to see this modeled.

   6. Draw the model and write the equation that goes with this subtraction problem (the equation that can be seen in the video).

2. Make two 3X5 units on the geoboard, then model 3/5 in one unit and 1/3 in the other unit.

   1. Place the island that models 1/3 over the islands that model 3/5.

   2. Decide how to cover the part that “sticks out” so you can name how much larger 3/5 is, compared to 1/3.

   3. Click here to check your decision.

   4. Draw your model and write the matching subtraction equation.

3. In a 4X6 unit model 5/6 minus 3/4 by first modeling 5/6 in the unit, then placing the model for 3/4 over it.

   1. Decide how to cover the part that represents the difference.

   2. Can you find two ways to cover it?

   3. Draw the models and write the matching subtraction equations.

4. Make a geoboard model, draw a picture on the Fraction Islands sheet, and write the equation for each of these examples:

   1. 1/2 – 2/7 in a 2X7 unit

   2. 8/9 – 2/3 in a 3X6 unit

   3. 1/4 – 1/6 in a 6X6 unit

1.