Number Talks: Adding and Subtracting

Lesson Starter

These two applets can be used as warm up activities to adding/subtracting to 10, 20 or 100.

link: https://www.geogebra.org/m/scs3wssn

link: https://www.geogebra.org/m/fxqrx2ae

Number Chats

Here are three smartboard/projector interactive applets to use with number talks about adding/subtracting.

Students might use small whiteboards to do their own calculations and drawings, or draw right on the main whiteboard.

Number Line

Change the max to 20 or to 100 for addition/subtraction to 20 or 100.

Change the min to a negative number if students are ready for negative numbers.

Chat 1: click all three red objects to show all three numbers and make ‘addition’ or ‘subtraction’ sentences with all three numbers.

Chat 2: click on two of the three red objects to show two numbers and ask students to calculate the third value.

Chat 3: give a value for the red line, and ask students to come up with a start and the end that will give the red line value.

Use this GeoGebra link to enable the full screen button.

Number Bar 20

This number bar has a maximum value of 20. Like the number line above, the diagram is drawn to scale to allow students to develop their sense of number alongside their sense of space.

As these diagrams are drawn to scale, it is possible to introduce some fraction vocabulary in the number talks, for example if the top bar is 20 and the given lower bar is 10, we might discuss that the bottom bars are equal and therefore are both half of 20.

It would be helpful to have a zero to 20 number line available to students and individual whiteboards during this number talk.

What is key in any number talk is to celebrate the methods the students use to figure out their solutions and not to proscribe a general method – ‘mathematizing’ is what all people do.

Use this GeoGebra link to enable the full screen option.

Number bar 100

It may be helpful to have a number line to 100 available to students to help them do their own calculations. As the top number can be as large as 100, there will no doubt be many different approaches to solution.

Be sure to look for and celebrate a diversity of approaches, including approaches that are mathematically correct even if the student ended up with the wrong answer.

Notice if student answers are in the right ‘ballpark’, even if they are not fully accurate – having 210 grams of rice will still satisfy a recipe that asks for 200 grams of rice. If students are using spatial sense alone they may not get the correct number but are still nonetheless engaged in productive mathematizing.

Use this GeoGebra link to enable the full screen option.