standard_description: Subtract multiples of 10 in the range 10-90 from multiples of 10 in the range 10-90 (positive or zero differences), 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 apply what they learned about rounding in prior lessons to think about all the numbers that would round to a given number. Students should be encouraged to use whatever representations make sense to them. Although the number line is often used to represent rounding, it is also worth sharing other ways that students are representing or thinking about rounding.
MLR8 Discussion Supports.
Synthesis: Some students may benefit from the opportunity to rehearse what they will say with a partner before they share with the whole class.
Advances: Speaking
Launch
Groups of 2
“Diego is thinking of a number. When you round Diego's number to the nearest ten, the answer is 40. What's a number that could be Diego's number? What's a number that could not be Diego's number?” (38 rounds to 40 so it could be his number. 34 does not round to 40 so it couldn't be his number.)
30 seconds: quiet think time
Share 3–5 responses. Highlight the idea that more than one number can round to 40 and that some numbers are greater than 40 and some are less than 40.
Activity
“Work with your partner on all these problems. Be sure to justify your reasoning.”
5–7 minutes: partner work time
Student Facing
What are all the numbers that would round to 50 if you're rounding to the nearest ten? You can use this number line if it helps you.
[IMG]
What are all the numbers that would round to 70 if you're rounding to the nearest ten?
What are all the numbers that would round to 600 if you're rounding to the nearest hundred?
If you finish early, find the numbers that would round to 100 and to 500 if you’re rounding to the nearest hundred. Compare your lists with a partner's lists and discuss patterns you see.
Student Response
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Advancing Student Thinking
If students don’t find all the numbers that round to the given number, consider asking:
“How did you determine that these numbers would round to ____?”
“How could you use a number line to find all the numbers that round to ___?”
Activity Synthesis
“How did you decide what numbers would round to 40?” (We looked at all the numbers that are closer to 40 than 50 or 30.)
Consider asking:
“What does 35 round to?” (40 because it is halfway between 30 and 40)
“What does 45 round to?” (50 because it is halfway between 40 and 50)
“Look at your responses for the first 2 problems. What patterns do you see in the numbers? Why is that happening?” (I see they each start with a 5 in the ones place below it because it’s halfway to the nearest ten, and the numbers end with a 4 in the ones place because that is closer than the next ten.)
“How did you use what you learned from the first 2 problems to think about the last problem?” (Instead of thinking about fives, we thought about fifties. We looked at all the numbers that are closer to 600 than 500 or 700.)
Consider asking:
“What does 550 round to?” (600 because it is halfway between 500 and 600.)
“What does 650 round to?” (700 because it is halfway between 600 and 700.)


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