Subtract Your Way

10 min

Narrative

The purpose of this Number Talk is to elicit the strategies and understandings students have for subtracting numbers within 1,000. These understandings help students develop fluency and will be helpful as students relate subtraction algorithms to strategies they have used to subtract within 1,000.

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.

  • 50−1050 - 10
  • 58−1058 - 10
  • 258−20258 - 20
  • 258−24258 - 24
Solution Steps (4)
  1. 1
    50 - 10
    →40 (5 tens - 1 ten = 4 tens)
  2. 2
    58 - 10
    →48 (same as 50-10=40, plus the 8 ones)
  3. 3
    258 - 20
    →238 (count back by tens: 258→248→238)
  4. 4
    258 - 24
    →234 (like previous, minus 4 more: 238-4=234)

Sample Response

  • 40: I knew that 5 tens minus 1 ten is 4 tens and 4 tens is 40.
  • 48: I knew that 50−1050 - 10 is 40, so this is just 8 more. 
  • 238: I counted back by tens 258, 248, 238.
  • 234: This is just like the last expression, but I took away 4 more.
Activity Synthesis (Teacher Notes)
  • “How was place value helpful as you subtracted these numbers?” (When we were subtracting 10, only the tens place changed. For the last expression we were able to subtract the tens, and then the ones.)
  • 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.

25 min

10 min