In this activity, students continue to work with ratios that involve one unit of something. They determine the prices of grocery items and learn to use the term unit price to describe cost per unit. To determine unit prices, students may:

• Divide the cost by the number of items.
• Use discrete diagrams.
• Use a double number line diagram.

As students work, monitor for students who use different methods. If students choose to draw a double number line diagram, remind them to label each number line and to circle the ratio where they find the answer.

In the digital version of the activity, students can choose to use an applet to create a double number line to find the unit price in different situations. The applet allows students to name the quantities, specify a ratio, and partition the double number line to show larger or smaller amounts that represent an equivalent ratio.

Select students who used unique methods to share their reasoning, as listed in the Activity Narrative. If no one used double number lines, represent one of the statements with a double number line diagram and display it for all to see. Although double number lines are not required in the task, their use in the context of problem situations helps students see their merits and illustrates how they might be used in other problems, especially as students transition from unit prices to constant speed and other contexts. Draw connections between the double number line strategy and the dividing by the numbers of items strategy.

Tell students that each “cost per one” unit being sold—avocado, pound, or bottle—is an example of a unit price. Ask them to name as many kinds of unit prices as they can and to think of a situation in which they might be used, starting with the list from the task:

• Cost per avocado
• Cost per pound
• Cost per bottle

Other possibilities include cost per liter, cost per ounce, cost per jelly bean, and so on.

In this activity, students practice finding unit prices, using different reasoning strategies, and articulating their reasoning. They also learn about the term “at this rate.”

For each problem, select 1–2 students or groups to share their solution and strategy later.

In the digital version of the activity, students can choose to use an applet to create a double number line to explore the problems.

Invite previously selected students or groups to share their work. Highlight accurate uses of the terms “at this rate” and “per,” and the ways in which a double number line might have been used when working with unit price.