standard_description: Add, subtract, multiply, and divide decimals to hundredths, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method and explain the reasoning used.
problem_activity: Narrative
The purpose of this activity is for students to make a reasonable estimate for a given product.
In addition to estimating the product, students also decide whether the estimate is too large or too small. In the activity synthesis, students consider about how far their estimate is from the actual product.
I
n the next activity, students will evaluate the expressions using a strategy of their choice.
Students choose between several different possible estimates and justify their choice before they calculate the product (MP3).
Engagement: Provide Access by Recruiting Interest.
Synthesis: Revisit math community norms to prepare students for the whole-class discussion.
Supports accessibility for: Social Emotional Functioning, Language
Launch
Groups of 2
Activity
5–7 minutes: independent work time
2–3 minutes: partner discussion
Monitor for students who:
relate the given expression to each proposed answer by rounding or changing one or both factors.
estimate by rounding the factors.
use benchmark numbers.
use place value reasoning or the properties of operations to explain why their estimate is reasonable.
Student Facing
Which estimate for the product
\(18 \times 149\)
is most reasonable? Explain or show your reasoning.
\(2,\!000\)
\(4,\!000\)
\(3,\!000\)
\(1,\!500\)
Are any of the estimates unreasonable? Explain or show your reasoning.
Do you think the actual product will be more or less than your estimate? Explain or show your reasoning.
Student Response
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Advancing Student Thinking
If students do not choose a reasonable estimate for
\(18 \times 149\)
, ask, “Can you explain how you chose your estimate?”
Activity Synthesis
Invite students to share their estimates for the product and their reasoning.
“Why do you think
\(3,\!000\)
is a good estimate?” (18 is close to 20 and 149 is very close to 150 and I know
\(20 \times 150 = 3,\!000\)
.)
“Is the value of
\(18 \times 149\)
greater than or less than
\(3,\!000\)
? How do you know?” (Less, because 18 is less than 20 and 149 is less than 150.)
“Can you estimate how much less?” (About 300, because I added 2 to 18 to get 20 and
\(2 \times 149\)
is about 300.)
Display:
\(18 \times 149\)
is about
\(2,\!700\)
.
“In the next activity, we’ll check to see how good our estimate is.”


aligns: yes