standard_description: Fluently add, subtract, multiply, and divide multi-digit decimals using the standard algorithm for each operation.
problem_activity: Activity
In this task, students practice finding unit prices, using different reasoning strategies, and articulating their reasoning. They also learn about the term “at this rate.”
As students work, observe their work and then assign one problem for each group to own and present to the class. (The problems can each be assigned to more than one group). Have them work together to create a visual display of their problem and its solution.
Teacher Notes for IM 6–8 Accelerated
Adjust this activity to 10 minutes.
In the launch, adjust the displayed problem to: “Pizza costs
$
10 for 8 slices. At this rate, how much will 6 slices cost?” During the discussion of the phrase “at this rate,” ask students to outline a method for solving the problem. Highlight any methods that determine the cost for 1 slice of pizza. Tell students that the price for 1 item is called the
unit price
and can be described using the word “per.” For example, “This pizza costs
$
1.25 per slice.”
Tell students to omit the last question in which they create a visual display.
Digital
Launch
Arrange students in groups of 3–4. Provide tools for creating a visual display and access to rulers. Explain that they will work together to solve some shopping problems, run their work by you, and prepare to present an assigned problem to the class. Tell students that they can use double number lines if they wish.
Display the problem and read it aloud: Pizza costs
$
1.25 per slice. At this rate, how much will 6 slices cost?
Ask students what they think “at this rate” means in the question. Ensure they understand that “at this rate” means we know that equivalent ratios are involved:
The ratio of cost to number of slices is
$
1.25 to 1. That is, pizza costs
$
1.25 per slice.
The ratio of cost to number of slices is
something
to 6. That is, pizza costs
something
for 6 slices.
The
something
is the thing we are trying to figure out, and “at this rate” tells us that the two ratios in this situation are equivalent. Another way to understand “at this rate” in this context is “at this price per unit” and that the price per unit is the same no matter how many items or units are purchased.
Discuss any expectations for the group presentation. For example, each group member might be assigned a specific role for the presentation.
If students have digital access, they can use an applet to explore the problems and justify their reasoning before preparing their presentations.
Student Facing
Four bags of chips cost
$
6.
What is the cost per bag?
At this rate, how much will 7 bags of chips cost?
At a used book sale, 5 books cost
$
15.
What is the cost per book?
At this rate, how many books can you buy for
$
21?
Neon bracelets cost
$
1 for 4.
What is the cost per bracelet?
At this rate, how much will 11 neon bracelets cost?
Pause here so your teacher can review your work.
Your teacher will assign you one of the problems. Create a visual display that shows your solution to the problem. Be prepared to share your solution with the class.
Here is an applet you may use if you choose to.
Student Response
Teachers with a valid work email address can
click here to register or sign in
for free access to Student Response.
Print
Launch
Arrange students in groups of 3–4. Provide tools for creating a visual display and access to rulers. Explain that they will work together to solve some shopping problems, run their work by you, and prepare to present an assigned problem to the class. Tell students that they can use double number lines if they wish.
Display the problem and read it aloud: Pizza costs
$
1.25 per slice. At this rate, how much will 6 slices cost?
Ask students what they think “at this rate” means in the question. Ensure they understand that “at this rate” means we know that equivalent ratios are involved:
The ratio of cost to number of slices is
$
1.25 to 1. That is, pizza costs
$
1.25 per slice.
The ratio of cost to number of slices is
something
to 6. That is, pizza costs
something
for 6 slices.
The
something
is the thing we are trying to figure out, and “at this rate” tells us that the two ratios in this situation are equivalent. Another way to understand “at this rate” in this context is “at this price per unit” and that the price per unit is the same no matter how many items or units are purchased.
Discuss any expectations for the group presentation. For example, each group member might be assigned a specific role for the presentation.
If students have digital access, they can use an applet to explore the problems and justify their reasoning before preparing their presentations.
Student Facing
Four bags of chips cost
$
6.
What is the cost per bag?
At this rate, how much will 7 bags of chips cost?
At a used book sale, 5 books cost
$
15.
What is the cost per book?
At this rate, how many books can you buy for
$
21?
Neon bracelets cost
$
1 for 4.
What is the cost per bracelet?
At this rate, how much will 11 neon bracelets cost?
Pause here so your teacher can review your work.
[IMG]
Your teacher will assign you one of the problems. Create a visual display that shows your solution to the problem. Be prepared to share your solution with the class.
Student Response
Teachers with a valid work email address can
click here to register or sign in
for free access to Student Response.
Anticipated Misconceptions
The first and third questions involve using decimals to represent cents. If the decimal point is forgotten, remind students that the cost of the bracelet is less than one dollar, and the cost of the chips is in between one and two dollars.
Watch for students working in cents instead of dollars for the bracelets. They may come up with an answer of 275 cents. For these students, writing 25 cents as \$0.25 should help, or consider reminding them of the avocados from a previous activity, which had a unit price of \$0.50.
Activity Synthesis
Invite each group to present its assigned problem. After each group presents, highlight the group’s strategy, 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.


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