Find the Perimeter

10 min

Narrative

The purpose of this Number Talk is to elicit strategies and understandings students have for dividing within 100. These understandings help students develop fluency and are helpful as students use division to solve problems involving perimeter.

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 strategy.
  • Keep expressions and work displayed.
  • Repeat with each expression.

Student Task

Find the value of each expression mentally.

  • 90÷990 \div 9
  • 81÷981 \div 9
  • 45÷945 \div 9
  • 54÷954 \div 9

Sample Response

  • 10: I just know it.
  • 9: 81 is 9 less than 90, so it is 1 fewer group of 9.
  • 5: It is half of 90÷990 \div 9 since 45 is half of 90.
  • 6: 54 is 9 more than 45, so it’s 1 more group of 9 than the previous expression.

Synthesis

  • “How could knowing 90÷990 \div 9 help you find the value of the other expressions?” (Once I knew 90÷990\div9, I was able to take away a group of 9 to find 81÷981\div9. I was able to find 45÷945\div9 by splitting the value of 90÷990 \div 9 because 45 is half of 90.)
Standards
Addressing
  • 3.OA.7·Fluently multiply and divide within 100, using strategies such as the relationship between multiplication and division (e.g., knowing that 8 × 5 = 40, one knows 40 ÷ 5 = 8) or properties of operations. By the end of Grade 3, know from memory all products of two one-digit numbers.
  • 3.OA.C.7·Fluently multiply and divide within 100, using strategies such as the relationship between multiplication and division (e.g., knowing that <span class="math">\(8 \times 5 = 40\)</span>, one knows <span class="math">\(40 \div 5 = 8\)</span>) or properties of operations. By the end of Grade 3, know from memory all products of two one-digit numbers.

20 min

15 min