Lesson 11

The purpose of this warm-up is to elicit observations and questions about pounds and ounces in a familiar context, which will be useful when students reason about the relationship between the 2 units later in the lesson.

• Groups of 2
• Display the image.
• “¿Qué observan? ¿Qué se preguntan?” // “What do you notice? What do you wonder?”
• 1 minute: quiet think time

• “Discutan con su compañero lo que pensaron” // “Discuss your thinking with your partner.”
• 1 minute: partner discussion
• Share and record responses. 

• “¿Pueden saber de cuál ingrediente necesitamos más y de cuál necesitamos menos?” // “Can you tell which ingredient we need more of and which we need less of?” (We can tell we need more rice than Cheddar cheese, or more kidney beans than beef, but it’s hard to tell of which item we need the largest or smallest amount because the ingredients are not all listed in the same unit.)
• “¿De cuál artículo se necesita la mayor cantidad? ¿De cuál se necesita la menor cantidad?” // “Which item requires the largest amount?  Which one requires the smallest amount?”

The goal of this activity is to build students’ intuition for 1 pound, 1 ounce, and the relationship between the two units. Students use labels on food packaging to reason about how pounds and ounces are related, and then use what they learn to convert pounds to ounces. Students reason abstractly and quantitatively when they determine the relationship between ounces and pounds from food labels (MP2).

Note that the quantities shown on food packaging or nutritional labels are often rounded rather than exact. For instance, a package labeled “32 ounces (2 pounds)” may contain 31.8 ounces but be labeled as 32 ounces or 2 pounds. In this activity, for the purposes of learning that one unit is 16 times the other, students will take the labels at face value. Consider explaining (during the activity synthesis) that, in reality, the label may not match the content exactly.

The blackline master includes images that can be used for the activity. If desired and if practical, actual packaged food could be used instead. In that case, be sure to choose items with whole numbers of pounds and ounces and to exclude items that show that 1 pound is equivalent to 16 ounces, as students reasoning about the equivalence is the point of the activity. Here are the images on the blackline master for reference:

• Create a set of cards from the blackline master for each group of 4.
• If possible, gather examples of packaged food items—one that is labeled “1 ounce” and another labeled “1 pound”.

• Groups of 3–4
• Display (or pass around, if possible) a 1-ounce pack and 1-pound bag of some food items.
• “¿Cómo creen que se relaciona 1 libra con 1 onza? ¿1 libra pesa cuántas veces lo que pesa 1 onza?” //  “How do you think 1 ounce compares to 1 pound? How many times as heavy is 1 pound compared to 1 ounce?”
• Collect and record estimates. 
• Give each group a set of images from the blackline master—one image per student.

• “Usen la información de las imágenes para descubrir cómo se relacionan 1 libra y 1 onza. Completen la actividad en grupo” // “Use the information on the images to find out how 1 pound and 1 ounce compare. Work with your group to complete the activity.”
• 8–10 minutes: group work time
• Monitor for:
  • the language students use to describe the relationship between pounds and ounces
  • the ways students reason about multiples of 16 as they complete the table (for instance, by adding 16 repeatedly, or by multiplying the number in pounds by 16, or a combination of the two)

• Invite selected students to share their conclusions about the relationship between pounds and ounces. 
• Highlight explanations that make it clear that 1 pound is 16 as much or as heavy as 1 ounce. 
• Display the table and invite students to share the amounts in ounces and their reasoning. 
• If needed, point out that the abbreviation “lb” is used for pounds and “oz” is used for ounces. 

In the first activity, students learned that one pound is 16 times as heavy as 1 ounce. Here they apply this knowledge to convert quantities into ounces and to solve multi-step problems. The quantities include a fractional number of pounds and one expressed in a combination of pounds and ounces.

As in earlier lessons in which they encountered a fractional amount of a unit of measurement, students are not expected to find the number of ounces in \(\frac{1}{2}\) pound by writing \(\frac{1}{2} \times 16\). Instead, they can reason about half of a quantity using their understanding of fractions and by dividing an amount by 2. Those who do write \(\frac{1}{2} \times 16\) represent the situation correctly, but this reasoning and the related operation will be developed in grade 5.

• Groups of 2
• “Usemos lo que sabemos sobre libras y onzas para resolver problemas” // “Let’s use what we know about pounds and ounces to solve problems.”

• 7–8 minutes: independent work time
• 3–4 minutes: partner discussion
• Monitor for different ways students reason about the last question.

• Select students to share their responses and reasoning. Record their responses for all to see.
• Clarify that sometimes both units are used to express a quantity that is not a whole number of pounds (as is the case with “2 pounds 8 ounces of pinto beans”).
• Select students who used different strategies to solve the last problem to share.