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.
4×40
8×40
7×40
9×40
Sample Response
160: I know 4×4 is 16, so 4×40 is 16 tens or 16×10, which is 160.
320: I know 8×40 is double 4×40, so it’s double 160, which is 320.
280: I know that 7×4=28, so this would be 28 tens, which is 280.
360: This would be two more forties than 7×40, so I just counted 280, 320, 360.
Synthesis
“How were you able to use 4×40 to find the other products.” (I doubled 4×40 to find 8×40. Once I knew 4×40, 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.