This Number Talk encourages students to look for and make use of structure to mentally evaluate a series of subtraction expressions. The numbers preview some benchmark angle measurements students will see in upcoming lessons.
Launch
Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
1 minute: quiet think time
Teacher Instructions
Record answers and strategies.
Keep expressions and work displayed.
Repeat with each expression.
Student Task
Find the value of each expression mentally.
90−45
270−45
270−135
360−135
Sample Response
45: 90=45+45
225: 270 is 180 more than 90, so 270−45 is 180 more than 90−45.
135: 135 is 90 more than 45, so 270−135 means subtracting an additional 90 from 270−45.
225: 360 is 90 more than 270, so 360−135 is 90 more than 270−135.
Activity Synthesis (Teacher Notes)
“What do these expressions have in common?” (The first number in each sequence is a multiple of 90.)
“How did this observation—that the first numbers are all multiples of 90—help you find the values of the differences?”
Consider asking:
“Who can restate _____’s reasoning in a different way?”
“Did anyone have the same strategy but would explain it differently?”
“Did anyone approach the expression in a different way?”
“Does anyone want to add on to _____’s strategy?”
Standards
Addressing
4.NBT.B·Use place value understanding and properties of operations to perform multi-digit arithmetic.
4.NBT.B·Use place value understanding and properties of operations to perform multi-digit arithmetic.
20 min
15 min
Knowledge Components
All skills for this lesson
No KCs tagged for this lesson
Points, Lines, Rays, and Segments
10 min
Narrative
This Number Talk encourages students to look for and make use of structure to mentally evaluate a series of subtraction expressions. The numbers preview some benchmark angle measurements students will see in upcoming lessons.
Launch
Display one expression.
“Give me a signal when you have an answer and can explain how you got it.”
1 minute: quiet think time
Teacher Instructions
Record answers and strategies.
Keep expressions and work displayed.
Repeat with each expression.
Student Task
Find the value of each expression mentally.
90−45
270−45
270−135
360−135
Sample Response
45: 90=45+45
225: 270 is 180 more than 90, so 270−45 is 180 more than 90−45.
135: 135 is 90 more than 45, so 270−135 means subtracting an additional 90 from 270−45.
225: 360 is 90 more than 270, so 360−135 is 90 more than 270−135.
Activity Synthesis (Teacher Notes)
“What do these expressions have in common?” (The first number in each sequence is a multiple of 90.)
“How did this observation—that the first numbers are all multiples of 90—help you find the values of the differences?”
Consider asking:
“Who can restate _____’s reasoning in a different way?”
“Did anyone have the same strategy but would explain it differently?”
“Did anyone approach the expression in a different way?”
“Does anyone want to add on to _____’s strategy?”
Standards
Addressing
4.NBT.B·Use place value understanding and properties of operations to perform multi-digit arithmetic.
4.NBT.B·Use place value understanding and properties of operations to perform multi-digit arithmetic.