Same Denominator or Numerator

10 min

Narrative

The purpose of this Number Talk is to elicit strategies and understandings students have for adding and subtracting multi-digit numbers. These understandings help students develop fluency and will be helpful in later units as students add and subtract multi-digit numbers fluently using the standard algorithm.

When students decompose addends to support mental addition, they are looking for and making use of the base-ten structure of numbers (MP7). 

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.

  • 136+100136 + 100
  • 136+300136 + 300
  • 136+370136 + 370
  • 136+378136 + 378

Sample Response

  • 236: 136+100=236136 + 100 = 236
  • 436: 136+300=436136 + 300 = 436
  • 506: This is 70 more than 436. I know 430+70=500430+70=500, so 436+70=506436+70=506.
  • 514: 300+100=400300+100=400, 30+70=10030+70=100, and 6+8=146+8=14, and 400+100+14=514400+100+14 =514. 

Synthesis

  • “How did the first couple of expressions help you reason about last two expressions?”
  • Consider asking:
    • “Who can restate _____’s reasoning in a different way?”
    • “Did anyone use 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
Building Toward
  • 4.NBT.4·Fluently add and subtract multi-digit whole numbers using the standard algorithm.
  • 4.NBT.B.4·Fluently add and subtract multi-digit whole numbers using the standard algorithm.

15 min

20 min