The lesson begins with a visual number talk asking students to determine the percent of area for each color in the figure. We are including a handout in case you want students to have a copy to fold, cut and/or notate. The goal is for them to be convincing and to share their methods and ideas. It is great if there are different answers because that makes for rich classroom discussion.
The next task is to determine which is better, taking a discount before the tax is added or after? The goal is for students to work in groups to make sense of the problem and determine a plan to test if their initial thinking was correct. Encourage students to write out their calculations and insights, use visuals, color coding and anything else that supports their thinking. Students should work through choosing their own examples, looking for patterns and then generalizing their work to any case.
The lesson plan and materials are provided in Google Slides— click below to view the slides or make your own copy of them. Slides labeled with a “T” in the upper right corner are included for teacher information about the lesson. Slides labeled with an “S” in the upper right corner are for students. If handouts are needed there are links to them in the teacher slides. All slides can be adapted.
Adapted from Thinking Mathematically by Mason, Burton and Stacey.