Attributes that Define Shapes

10 min

Narrative

This Number Talk prompts students to use place value and properties of operations to multiply single-digit numbers by multiples of ten. The strategies elicited here help students develop fluency.

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.

  • 4×404\times40
  • 8×408\times40
  • 7×407\times40
  • 9×409\times40

Sample Response

  • 160: I know 4×44 \times 4 is 16, so 4×404\times 40 is 16 tens or 16×1016 \times 10, which is 160.
  • 320: I know 8×408 \times 40 is double 4×404 \times 40, so it’s double 160, which is 320.
  • 280: I know that 7×4=287\times4=28, so this would be 28 tens, which is 280.
  • 360: This would be two more forties than 7×407\times40, so I just counted 280, 320, 360.
Activity Synthesis (Teacher Notes)
  • “How were you able to use 4×404\times40 to find the other products.” (I doubled 4×404\times40 to find 8×408\times40. Once I knew 4×404\times 40, I could count up or down by 40 to get the other products.)
  • Consider asking: “Why might it be helpful to first think of each expression as multiplying a number and 4 instead of 40?” (We know many multiplication facts for 4, and that 40 is 4 tens. We can think of each problem as groups of 4 tens instead of groups of 40.)
Standards
Addressing
  • 3.NBT.3·Multiply one-digit whole numbers by multiples of 10 in the range 10—90 (e.g., 9 × 80, 5 × 60) using strategies based on place value and properties of operations.
  • 3.NBT.A.3·Multiply one-digit whole numbers by multiples of 10 in the range 10–90 (e.g., <span class="math">\(9 \times 80\)</span>, <span class="math">\(5 \times 60\)</span>) using strategies based on place value and properties of operations.

10 min

25 min