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.
Synthesis
“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.