8th Grade Mathematics Lesson

Phone Charger Race: Discovering Slope and Graphing Linear Equations

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8th GradeThin-SlicingCCSS.MATH.CONTENT.8.EE.B.5, CCSS.MATH.CONTENT.8.EE.B.650 min

Learning Objective

Students will be able to calculate slope from tables, graphs, and equations, identify the y-intercept, and graph linear equations in slope-intercept form (y = mx + b) by connecting each parameter to a real-world charging rate context. (CCSS.MATH.CONTENT.8.EE.B.5, 8.EE.B.6)

Lesson Overview

Students use a phone-charging context to discover how slope (rate of change) and y-intercept work together in y = mx + b, progressing from reading rates on a table to graphing and comparing linear equations with different slopes and intercepts.

Materials

Vertical whiteboards or chart paper
Dry-erase markers (multiple colors)
Rulers or straight edges
Random grouping tool
Printed task cards (optional)
Graphing calculators or Desmos access (optional for extension)

Scaffolded Task Progression

1Task 1

Phone A charges at a steady rate. The table below shows its battery percentage over time.

Time (min)	0	1	2	3	4
Battery %	10	12	14	16	18

How much does the battery increase each minute? What was the starting battery level?

2Task 2

Phone B also charges steadily. Here is its table:

Time (min)	0	1	2	3	4
Battery %	40	41	42	43	44

How much does Phone B's battery increase each minute? What is its starting level? How is Phone B different from Phone A (Task 1)?

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