Interpret Division Expressions

10 min

Narrative

The purpose of this Number Talk is to elicit strategies and understandings students have for subtracting within 1,000, particularly around adding up to find differences. These understandings help students develop fluency for subtracting within 1,000.

Launch

  • Display the first 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.

  • 500−475500 - 475
  • 504−475504 - 475
  • 512−475512 - 475
  • 512−449512 - 449
Solution Steps (4)
  1. 1
    Calculate 500-475
    →Count up: 475+25=500, so difference is 25
  2. 2
    Calculate 504-475
    →Same as above plus 4 more: 25+4=29
  3. 3
    Calculate 512-475
    →475+25=500, +10=510, +2=512, so 25+10+2=37
  4. 4
    Calculate 512-449
    →449+51=500, +12=512, so 51+12=63

Sample Response

  • 25: I added 25 to 475 to get to 500.
  • 29: 29 is 4 more than 25. So, I added 4 to the sum in the first problem to get to 504.
  • 37: I started at 475 and added 25 to get to 500, then I added 10 to get 510, and 2 more to get to 512. 25+10+2=3725 + 10 + 2 = 37.
  • 63: I know that 49 is 51 away from 500 and then I added 12 more.
Activity Synthesis (Teacher Notes)
  • ”Why is the value of 512−475512 - 475 greater than the value of 504−475504 - 475?” (Since the 475 doesn’t change and 512 is greater than 504, the difference between 512 and 475 is greater.)
  • 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 problem in a different way?”
    • “Does anyone want to add on to _____’s strategy?”
Standards
Addressing
  • 3.NBT.2·Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction.
  • 3.NBT.A.2·Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction.

10 min

10 min

15 min