Use Strategies and Algorithms to Add

10 min

Narrative

The purpose of this Number Talk is to elicit the strategies and understandings students have for adding within 1,000. These understandings help students develop fluency and will be helpful later in this lesson when students decide whether to use an algorithm or another strategy to add.

When students notice that a number is close to a multiple of 100 and use this to add, they are looking for and making use of structure (MP7).

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.

  • 300+156300 + 156
  • 299+156299 + 156
  • 303+156303 + 156
  • 204+376204 + 376
Solution Steps (4)
  1. 1
    300 + 156
    →456 (add 3 hundreds to 156)
  2. 2
    299 + 156
    →455 (299 is 1 less than 300, so answer is 456-1=455)
  3. 3
    303 + 156
    →459 (303 is 3 more than 300, so answer is 456+3=459)
  4. 4
    204 + 376
    →580 (200+376=576, then add 4 more = 580)

Sample Response

  • 456: 300+156300+156. I added hundreds to 156 to make it 456.
  • 455: I noticed that it was just like the first problem, but 1 less, so instead of 456, it’s 455.
  • 459: I noticed that it was just like the first problem, but 3 more, so instead of 456, it’s 459.
  • 580: I started by adding 200 to 376 because it was easy to add more hundreds to get 576. Then I added the extra 4, which made 580.
Activity Synthesis (Teacher Notes)
  • “What is it that made these numbers easier to add mentally?” (The first numbers in the first 3 problems were really close to 300, so we were able to add 300 and make little adjustments. In the last problem, the first number was really close to 200, which made it easy to subtract mentally.)
  • Consider asking:
    • “Who can restate _______ 's reasoning in a different way?”
    • “Did anyone have the same strategy but would explain it differently?”
    • “Did anyone approach the problem in a different way?”
    • “Does anyone want to add on to____’s strategy?”
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.

15 min

20 min