In this Warm-up, students examine satellite images that show how the size of the Aral Sea has changed over time. This prepares them for later in the lesson when they will calculate percentages that represent the situation.

Invite several partners to share one question with the class and record responses. Ask the class to make comparisons among the shared questions and their own. Ask, “What do these questions have in common? How are they different?” Listen for and amplify language related to the learning goal, such as “area,” “percent increase,” and “percent decrease”.

Explain that these are satellite photos of the Aral Sea, in central Asia. The amount of water in the sea has been decreasing since the 1960s when two rivers, the Syr Darya and the Amu Darya, were diverted to irrigate land to grow cotton and other crops. The Aralkum Desert is taking over the land where the Aral Sea used to be. Invite students to brainstorm how these changes may be affecting the people that live in this region.

In this activity, students examine two tables of information related to the Aral Sea. They choose what pairs of values to compare, and they calculate the percent increase or decrease between the values. They write sentences describing what each percentage represents about the situation. As students choose what aspects of the data to focus on for their calculations,  they are making sense of the problem (MP1). As students clearly state what each percentage represents, about the situation, they are attending to precision (MP6).

In this activity students apply reasoning about percent increase and decrease to information about the Great Barrier Reef. They calculate the percentage by which the coral coverage has decreased. They also calculate what the new area would be after a potential percent increase.

In the Are You Ready for More? task students explore a repeated percent increase. Although students have not learned about compounded rates, they can continue multiplying the area by 1.0089 until they reach the desired value, tracking the number of times they multiply. Students attempting this process may benefit from being shown how most calculators will use the result of the previous calculation and repeat the same operation when the “=” button is pressed repeatedly.