Divide Decimals by Decimals

10 min

Narrative

The purpose of this Number Talk is to elicit strategies and understandings students have to divide whole numbers by decimals. These understandings help students develop fluency and will be helpful later in this lesson when students divide decimals greater than 1 by decimals less than 1.

Launch

  • Display one problem.
  • “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 problems and work displayed.
  • Repeat with each problem.

Student Task

Find the value of each expression mentally.

  • 20÷220 \div 2
  • 2÷0.22 \div 0.2
  • 50÷250 \div 2
  • 5÷0.25 \div 0.2

Sample Response

  • 10: 10×2=2010 \times 2 = 20
  • 10: 1÷0.2=51 \div 0.2 = 5, 2×5=102 \times 5 = 10
  • 25: I just know it.
  • 25: I'm dividing 50 tenths into groups of 2 tenths so that's the same as 50÷250 \div 2.

Synthesis

  • “Why do 50÷250 \div 2 and 5÷0.25 \div 0.2 have the same value?” (Because 5 is 50 tenths and I am dividing that into groups of 2 tenths. So, that's 50÷250 \div 2.)
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