Rectangles with the Same 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 will be helpful later in this lesson when students will need to be able to divide fluently within 100.

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.

  • 5×55 \times 5
  • 10×510 \times 5
  • 2×52 \times 5
  • 85÷585 \div 5

Sample Response

  • 25: I just know it or it’s 5 groups of 5.
  • 50: I just know it or it’s 10 groups of 5.
  • 10: I just know it.
  • 17: I know that 50+25+10=8550 + 25 + 10 = 85. There are 5 fives in 25, 10 fives in 50, and 2 fives in 10. 5+10+2=175 + 10 + 2 = 17. So, there are 17 fives in 85.
Activity Synthesis (Teacher Notes)
  • “How did knowing the first 3 facts help you find the value of 85÷585 \div 5?” (The first 3 products added up to 85, which was the number I was dividing in the last problem. So, I was able to use those to figure out 85÷585 \div 5.)
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.

15 min

20 min