Lesson 4

In this second lesson about percent increase and percent decrease, students work with problems where they are given the final amount after a percent increase or decrease and must calculate the original amount, or are given the final and original amounts and must calculate the percent increase or decrease. They use double number lines to visualize such situations in order to help see clearly which of the two amounts involved, the starting amount or the final amount, is to be regarded as the whole, or 100%. They explore common misconceptions resulting from getting confused about which amount is the whole. For example, if you are given the final amount after a 10% decrease, a common error is to regard that final amount as the whole and and calculate the original amount by adding 10% of the final amount. Being clear about which quantity is the whole is a good example of attending to precision (MP6).