standard_description: Rearrange formulas to highlight a quantity of interest, using the same reasoning as in solving equations. For example, rearrange Ohm’s law V = IR to highlight resistance R.
problem_activity: Activity
This activity allows students to practice reasoning about situations involving ratios of two quantities and their sum. It also introduces students to using “parts” in recipes (e.g., 3 parts oil with 2 parts soy sauce and 1 part orange juice), instead of more familiar units such as cups, teaspoons, milliliters, etc. Students may use tape diagrams to support their reasoning, or they may use other representations learned so far—discrete diagrams, number lines, tables, or equations. All approaches are welcome as long as students use them to represent the situations appropriately to support their reasoning.
As students work, monitor for different ways students reason about the problems, with or without using tape diagrams.
Launch
Keep students in the same groups. Provide graph paper and snap cubes (any three colors). Explain that they will now practice solving problems involving ratios and their combined quantities (similar to the green and purple paint in the previous task). Draw students to a ratio that uses “parts” as its unit. Ask students what they think “one part” means or amounts to, and how situations expressed in terms of “parts” could be diagrammed.
Before students begin working, make sure they understand that “parts” do not represent specific amounts, that the value of “one part” can vary but the size of all parts is equal, and that a tape diagram can be used to show these parts.
Student Facing
Solve each of the following problems and show your thinking. If you get stuck, consider drawing a
tape diagram
to represent the situation.
The ratio of students wearing sneakers to those wearing boots is 5 to 6. If there are 33 students in the class, and all of them are wearing either sneakers or boots, how many of them are wearing sneakers?
A recipe for chicken marinade says, “Mix 3 parts oil with 2 parts soy sauce and 1 part orange juice.” If you need 42 cups of marinade in all, how much of each ingredient should you use?
Elena makes fruit punch by mixing 4 parts cranberry juice to 3 parts apple juice to 2 parts grape juice. If one batch of fruit punch includes 30 cups of apple juice, how large is this batch of fruit punch?
Student Response
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Student Facing
Are you ready for more?
Using the recipe from earlier, how much fruit punch can you make if you have 50 cups of cranberry juice, 40 cups of apple juice, and 30 cups of grape juice?
Student Response
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Anticipated Misconceptions
Students may think of each segment of a tape diagram as representing each cube, rather than as a flexible representation of an increment of a quantity. Help them set up the tapes with the correct number of sections and then discuss how many parts there are in all.
Activity Synthesis
Select students to share their reasoning. Help students make connections between different representations, especially any tape diagrams.


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