Previously, students wrote expressions for percentage increase or decrease using multiplication only. In this lesson, students explore applying percentage increase or decrease repeatedly. Students see examples showing that repeatedly applying the percent change to the original amount is not the same as repeatedly applying it to each new amount. When students explain this common mistake, they are critiquing the reasoning of others (MP3).

In one activity students play a game and calculate cumulative increases or decreases of running game scores. Then students look at the common error and explore the difference when applied incorrectly. This lesson helps prepare students for the associated Algebra I lesson involving compound interest and writing an expression in the form to represent a percentage increase applied times.