Estimate and Subtract

10 min

Narrative

The purpose of this Number Talk is for students to develop mental strategies for subtracting decimals where decomposition of one or more units is needed when subtracting by place value. These strategies will be useful as students subtract more complex decimals and need to pay close attention to place value.

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.

  • 1.50−0.511.50 - 0.51
  • 1.50−0.521.50 - 0.52
  • 1.50−0.601.50 - 0.60
  • 1.50−0.621.50 - 0.62

Sample Response

  • 0.99: 1.50−0.50=11.50-0.50=1, 1−0.01=0.991-0.01=0.99
  • 0.98: I subtracted one more hundredth from the first problem.
  • 0.90: 1.50−0.50=11.50-0.50=1, 1−0.10=0.901-0.10=0.90
  • 0.88: I subtracted 2 more hundredths from the third problem.
Activity Synthesis (Teacher Notes)
  • “How did you use the value of the third expression to find the value of the last expression?” (I know 0.62 is two hundredths more than 0.60 so I subtracted 2 hundredths from 0.90 and that left 0.88.)
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