Making a Shelf for Matchbox Cars

The teacher will ask, “What do you notice?” The teacher wants to highlight the observations involving 8 cars on each shelf and there are 3 shelves (Slide 4).

The teacher will hold up a measuring tape or a centimeter cube. The teacher will hold up the Matchbox car. For the purposes of our problem, we are going to agree that each car is about 7 centimeters long (Slide 5).

The teacher will place the students with a partner. Although the length of the cars may vary slightly, for the purposes of problem-solving, students will use 7 centimeters as the standard length of a typical Matchbox car. As the students work, look to see who finds the length of one row and then adds the length three times and who counts all (adds 7 twenty-four times or counts all by ones). The teacher will ask each partnership to estimate how many centimeters of wood they will need to build three shelves. This helps them evaluate the reasonableness of their final answer. The partnerships will record their estimate on their recording sheet. To help the students solve the problem, provide them with either centimeter cubes, centimeter paper, several paper rulers, regular rulers, or measuring tape. Students may need paper clips to mark seven centimeters on the standard measuring tool (measuring tape or printed rulers) of their choice (Slide 6). 

Distribute a recording sheet to each partnership. Quickly review with the students the parts of the recording sheet (Slide 7).

Review the problem the students will solve. This slide needs to be projected at all times or print copies of this slide to give to each partnership. While the students work, the teacher will circulate and ask prompting and probing questions. “What do you know? How will you use that information? What measurement unit are we using in the problem? What addition strategy could you use to help you add multiple addends? How does that strategy change or stay the same when the addends are the same number being added over and over?” (Slide 8).

Choose three partnerships to share. The lowest level shares first. The highest level shares last. The complexity of strategy may vary due to skill level or the time of the school year, so each teacher will have to look at their individual class to rank strategy complexity to determine what is the lowest level and what is the highest level. Typically,  the lowest level will count on by 7 each time. The highest level will determine the length of one row (56 cm), then add 56 cm three times, decomposing by place value to add 56 + 56 + 56. The additive framework may be helpful when evaluating work (Slide 9).

Review the problem the students will solve. Keep the problem posted as students share their work. As the students share, use the checklist to formatively evaluate their work (Slide 10).