Find the Area of a Figure

10 min

Narrative

The purpose of this Number Talk is to elicit strategies students have for adding two numbers when one number is close to a ten. These understandings help students develop fluency in addition. Students may look for and make use of structure (MP7) in a number of ways. For example, they may add 1 to the first addend to make a full ten and subtract 1 from the second addend to find each sum. They also may notice how the addends compare to those in the previous expression and use the change to find the new sum.

In this string, students also may add the tens and the ones separately to find the sum. Adding by place value is the focus of upcoming work. This Number Talk also enables the teacher to learn the strategies students currently use for addition.

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.

  • 109+4109 + 4
  • 109+14109 + 14
  • 209+34209 + 34
  • 219+34219 + 34
Solution Steps (4)
  1. 1
    Solve 109+4 using make-a-ten strategy
    →Take 1 from 4 to make 110, add remaining 3: 110+3=113
  2. 2
    Solve 109+14 by comparing to previous
    →10 more than 113, so 123
  3. 3
    Solve 209+34 using make-a-ten strategy
    →Take 1 from 34 to make 210, add remaining 33: 210+33=243
  4. 4
    Solve 219+34 by comparing to previous
    →10 more than 243, so 253

Sample Response

  • 113: I took away 1 from the 4 and added it to the 109 to make 110, and then added the remaining 3.
  • 123: I knew it was 10 more than the first sum.
  • 243: I took away 1 from the 34 and added it to the 209 to get 210, and then added the 33 by counting 210, 220, 230, 240, and then 243.
  • 253: I noticed it’s 10 more than the last sum because we started at 219 instead of 209.
Activity Synthesis (Teacher Notes)
  • “What do you notice about how the first two expressions are related? What about how the last two expressions are related?” (The second sum is 10 more than the first. The fourth sum is 10 more than the third.)
Standards
Addressing
  • 3.NBT.2·Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction.
  • 3.NBT.A.2·Fluently add and subtract within 1000 using strategies and algorithms based on place value, properties of operations, and/or the relationship between addition and subtraction.

20 min

15 min