standard_description: Understand the concept of a ratio and use ratio language to describe a ratio relationship between two quantities. For example, “The ratio of wings to beaks in the bird house at the zoo was 2:1, because for every 2 wings there was 1 beak.” “For every vote candidate A received, candidate C received nearly three votes.”
problem_activity: Activity
The purpose of this activity is to give students practice using the language of percent error. The problems here are similar in structure to percent increase or decrease problems, but the language is different. Students may need some help interpreting the language used for percent error and drawing parallels to the language used for percentage increase and decrease. Students should use similar strategies they used to calculate percentage increase or decrease. This activity includes one of each type of problem:
finding the erroneous amount given the correct amount and the percent error
finding the correct amount given the erroneous amount and the percent error
finding the percent error given the erroneous amount and the correct amount
Students may need help understanding that a different approach is needed for each question. As students work, monitor for those who use strategies similar to those used for percentage increase and decrease, and ask them to share during the whole-class discussion.
Launch
Before students begin working, read the first question aloud. It is helpful to tie the language used in the task to the phrase "percentage increase." Ask several students to explain in their own words what information is given in the problem, and what it is asking them to find. The plant is supposed to get
\(\frac34\)
cup of water, but it is getting 25% more than that. We can think of this as
\(\frac34\)
cup increased by 25%.
Arrange students in groups of 2. Give students 3–5 minutes of quiet work time. Partner then by whole-class discussion.
Representation: Internalize Comprehension.
Activate or supply background knowledge. Represent the same information through different modalities. If students are unsure where to begin, suggest that they draw a double number line to help organize the information provided.
Supports accessibility for: Conceptual processing; Visual-spatial processing
Student Facing
Instructions to care for a plant say to water it with
\(\frac34\)
cup of water every day. The plant has been getting 25% too much water. How much water has the plant been getting?
The pressure on a bicycle tire is 63 psi. This is 5% higher than what the manual says is the correct pressure. What is the correct pressure?
The crowd at a sporting event is estimated to be 3,000 people. The exact attendance is 2,486 people. What is the
percent error
?
Student Response
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Student Facing
Are you ready for more?
A micrometer is an instrument that can measure lengths to the nearest micron (a micron is a millionth of a meter). Would this instrument be useful for measuring any of the following things? If so, what would the largest percent error be?
The thickness of an eyelash, which is typically about 0.1 millimeters.
The diameter of a red blood cell, which is typically about 8 microns.
The diameter of a hydrogen atom, which is about 100 picometers (a picometer is a trillionth of a meter).
Student Response
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for free access to Extension Student Response.
Anticipated Misconceptions
Students might struggle with figuring out how to calculate how much water the plant has been getting. Ask students, "How much more water has the plant been getting? How do you calculate that total?"
Activity Synthesis
Select students who used these strategies for each problem to share:
Calculate a quarter of
\(\frac34\)
and add it to
\(\frac34\)
.
Use 1.05 to divide into 63.
Calculate percent error using
\(\frac{514}{2,486}\)
.
After each student has shared, ask the class if they agree or disagree or if they had a different way to calculate the solution. If students use strategies similar to ones they did calculating percentage increase or decrease, ask students if they see a connection. If no student brings it up, ask students how the solution strategies here are similar to the ones used with percentage increase and decrease.
Speaking: MLR8 Discussion Supports.
As students describe their strategies for calculating the erroneous amount, correct amount, and percent error, revoice student ideas to demonstrate mathematical language use. Press for details in students’ explanations by requesting that students challenge an idea, elaborate on an idea, or give an example. This will help students to produce and make sense of of the language needed to communicate their own ideas.
Design Principle(s): Support sense-making; Optimize output (for explanation)


aligns: no