standard_description: Interpret multiplication as scaling (resizing), by:
- Explaining why multiplying a given number by a fraction greater than 1 results in a product greater than the given number (recognizing multiplication by whole numbers greater than 1 as a familiar case); explaining why multiplying a given number by a fraction less than 1 results in a product smaller than the given number; and relating the principle of fraction equivalence a/b = (n×a)/(n×b) to the effect of multiplying a/b by 1.
- Comparing the size of a product to the size of one factor on the basis of the size of the other factor, without performing the indicated multiplication.
problem_activity: Activity
In this activity, students identify what equivalent ratios have in common (a ratio equivalent to
\(a:b\)
can be generated by multiplying both
\(a\)
and
\(b\)
by the same number) and generate equivalent ratios (MP8). It is at this point in the unit where students will explicitly define the term equivalent ratios (MP6).
Launch
Arrange students in groups of 3–4. Provide each group with tools for creating a visual display.
Summarize what we know so far about equivalent ratios. When we double or triple a color recipe, the ratios of the amount of ingredients in the mixtures are equivalent to those in the original recipe. For example,
\(24:9\)
and
\(8:3\)
are equivalent ratios, because we can think of
\(24:9\)
as a mixture that contains three batches of purple water where a single batch is
\(8:3\)
.
When we make multiple batches of a food recipe, we say the ratios of the amounts of the ingredients are equivalent to the ratios in a single batch. For example,
\(3:6\)
,
\(1:2\)
, and
\(9:18\)
are equivalent ratios because they correspond to the amount of the ingredients in different numbers of batches of tuna noodle casserole, and they all taste the same.
In this activity, we’ll write a definition for
equivalent ratios
.
When students pause after question 5, have a whole class discussion about the first five questions. Then assign each group a different ratio to use as their example for their visual display. Some possibilities:
\(4:5\)
\(3:2\)
\(5:6\)
\(3:4\)
\(2:5\)
Writing and speaking: Math Language Routine 1 Stronger and Clearer Each Time.
This is the first time Math Language Routine 1 is suggested as a support in this course. In this routine, students are given a thought-provoking question or prompt and asked to create a first draft response. Students meet together in 2–3 partners to share and refine their response through conversation. While meeting, listeners ask questions such as: “What did you mean by . . .?” and “Can you say that another way?” Finally, students write a second draft of their response that reflects ideas from their partners, and improvement on their writing. The purpose of this routine is to provide a structured and interactive opportunity for students to revise and refine their ideas through verbal and written means.
Design Principle(s): Optimize output (for generalization)
How It Happens:
Use this routine to provide students a structured opportunity to revise and refine their response to “How do you know when ratios are equivalent and when they are not equivalent?” Allow students 2–3 minutes to individually create first draft responses.
Invite students to meet with 2–3 other partners for feedback.
Instruct the speaker to begin by sharing their ideas without looking at their written draft, if possible. Provide the listener with these prompts for feedback that will help teams strengthen their ideas and clarify their language: “Can you explain how...?”, “You should expand on….”, “Can you give an example of equivalent ratios?”, and “Could you justify that differently?” Be sure to have the partners switch roles. Allow 1–2 minutes to discuss.
Signal for students to move on to their next partner and repeat this structured meeting.
Close the partner conversations and invite students to revise and refine their writing in a second draft. Students can borrow ideas and language from each partner to strengthen the final product.
Provide these sentence frames to help students organize their thoughts in a clear, precise way: “I know when ratios are equivalent/not equivalent when….” and “An example of this is…because….”
Here is an example of a second draft:
“I know when ratios are equivalent when I multiply both parts of one ratio by the same number and I get the other ratio. For example, I know that
\(5:3\)
is equivalent to
\(30:18\)
because when I multiply 5 by 6, I get 30, and when I multiply 3 by 6, I get 18, so 6 is the same number used to multiply both parts. But
\(5:3\)
is not equivalent to
\(15:12\)
because when I multiply 5 by 3, I get 15, and when I multiply 3 by 4, I get 12. So since a different number is used to multiply to get the second ratio, they’re not equivalent.”
If time allows, instruct students to compare their first and second drafts. If not, the students can continue on with the lesson by returning to their first partner and creating the visual.
Student Facing
The ratios
\(5:3\)
and
\(10:6\)
are
equivalent ratios
.
Is the ratio
\(15:12\)
equivalent to these? Explain your reasoning.
Is the ratio
\(30:18\)
equivalent to these? Explain your reasoning.
Give two more examples of ratios that are equivalent to
\(5:3\)
.
How do you know when ratios are equivalent and when they are
not
equivalent?
Write a definition of
equivalent ratios
.
Pause here so your teacher can review your work and assign you a ratio to use for your visual display.
Create a visual display that includes:
the title “Equivalent Ratios”
your best definition of
equivalent ratios
the ratio your teacher assigned to you
at least two examples of ratios that are equivalent to your assigned ratio
an explanation of how you know these examples are equivalent
at least one example of a ratio that is
not
equivalent to your assigned ratio
an explanation of how you know this example is
not
equivalent
Be prepared to share your display with the class.
Student Response
Teachers with a valid work email address can
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for free access to Student Response.
Anticipated Misconceptions
Students may incorporate recipes, specific examples, or batch thinking into their definitions. These are important ways of thinking about equivalent ratios, but challenge them to come up with a definition that only talks about the numbers involved and not what the numbers represent.
If groups struggle to get started thinking generally about a definition, give them a head start with: “A ratio is equivalent to
\(a:b\)
when . . .”
If students include “or divide” in their definition, remind them that, for example, dividing by 5 gives the same result as multiplying by one-fifth. Therefore, we can just use “multiply” in our definition.
Activity Synthesis
Each group will share their visual display as they explain their definitions. Highlight phrases or explanations that are similar in each display. Make one class display that incorporates all valid definitions. This display should be kept posted in the classroom for the remaining lessons within this unit. It should look something like:
Equivalent Ratio
A ratio is equivalent to
\(a:b\)
when both
\(a\)
and
\(b\)
are multiplied by the same number.


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