Addition and Subtraction

10 min

Narrative

The purpose of this Number Talk is to find the value of subtraction expressions with decimal numbers which encourage adding on as a strategy. While the first two expressions can be found readily by taking away, finding the difference or adding on to the smaller number is an effective strategy. The second pair of expressions also encourage adding on or compensation strategies. Students will have an opportunity to use these strategies in the lesson as they continue to add and subtract decimals.

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.

  • 2.57−2.552.57 - 2.55
  • 2.57−2.492.57 - 2.49
  • 2.57−0.992.57 - 0.99
  • 2.57−0.592.57 - 0.59

Sample Response

  • 0.02: They're the same except 2.57 has 2 more hundredths.
  • 0.08: I took away 7 hundredths to get 2.50 and then 1 more.
  • 1.58: I added 0.01 to get 1 and then 1.57 more to get 2.57.
  • 1.98: I added 4 tenths to the previous answer. I added 2 and then took away 2 hundredths.
Activity Synthesis (Teacher Notes)
  • “What strategy did you use to find the value of 2.57−0.992.57 - 0.99?” (I subtracted 1.57 to get 1 and then one more hundredth to get 0.99. I added 0.01 to 0.99 to get 1 and then added 1.57 more to get 2.57.)
Standards
Addressing
  • 5.NBT.7·Add, subtract, multiply, and divide decimals to hundredths, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method and explain the reasoning used.
  • 5.NBT.B.7·Add, subtract, multiply, and divide decimals to hundredths, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method and explain the reasoning used.

15 min

10 min