Estimate and Add

10 min

Narrative

The purpose of this Number Talk is for students to develop strategies for adding decimals with a focus on composing a new whole number. Students will work on sums where a new whole number is composed throughout the lesson.

Launch

  • Display one expression.
  • “Give me a signal when you have an answer and can explain how you got it.”
Teacher Instructions
  • 1 minute: quiet think time
  • Record answers and strategy.
  • Keep expressions and work displayed.
  • Repeat with each expression.

Student Task

Find the value of each expression mentally.

  • 1.00+0.99+0.021.00 + 0.99 + 0.02
  • 1.99+0.021.99 + 0.02
  • 1.99+0.031.99 + 0.03
  • 1.99+0.131.99 + 0.13

Sample Response

  • 2.01: I added one of the hundredths from 0.02 to 0.99 to make another whole and then I had 2 and one hundredth.
  • 2.01: It is the same problem as the first one, but 1.00+0.991.00 + 0.99 is written as 1.99.
  • 2.02: It is 1 hundredth more than the last one.
  • 2.12: It is 1 tenth more than the last one and 2.02+.1=2.122.02 + .1 = 2.12.

Synthesis

  • “How did you use the value of one of the expressions to help you find the value of a different expression?” (I knew that 1.99+0.131.99 + 0.13 is 0.1 more than 1.99+0.031.99 + 0.03.)
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

20 min