Multiplication for Equal Groups

10 min

Narrative

The purpose of this Number Talk is to elicit strategies and understandings students have for addition within 100. It also provides an opportunity to observe student strategies as they work toward becoming fluent in addition within 1,000.

When students use strategies based on place value to add, they look for and make use of structure (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.

  • 40+3540+35
  • 45+3545+35
  • 45+3645+36
  • 34+5834+58
Solution Steps (4)
  1. 1
    Find 40+35 using place value
    →4 tens + 3 tens = 7 tens, plus 5 ones = 75
  2. 2
    Find 45+35 using compose-a-ten strategy
    →7 tens + 5 + 5 = 7 tens + 10 = 80
  3. 3
    Find 45+36 by relating to previous
    →One more than 45+35, so 80 + 1 = 81
  4. 4
    Find 34+58 using place value and regrouping
    →8 tens + 12 ones = 9 tens + 2 ones = 92

Sample Response

  • 75: There are 7 tens and 5 ones, which is 75.
  • 80: There are 7 tens and 5 ones plus 5 makes another ten, which would be 80.
  • 81: It's like the one right before it, but there's 1 more, which would make 81.
  • 92: There are 8 tens and 12 ones. If I break apart the 12 into 1 ten and 2 ones, then I have 9 tens and 2 ones, which is 92.
Activity Synthesis (Teacher Notes)
  • "How did you compose new tens as you solved these problems?" (In the second problem I composed a ten from the 2 fives. In the third problem I composed a new ten from the 5 ones and the 6 ones and still had 1 one leftover.)
  • 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 problem in a different way?"
    • "Does anyone want to add on to____'s strategy?"
Standards
Building On
  • 2.NBT.5·Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction.
  • 2.NBT.B.5·Fluently add and subtract within 100 using strategies based on place value, properties of operations, and/or the relationship between addition and subtraction.

15 min

20 min