Analyze Addition Mistakes

10 min

Narrative

The purpose of this Estimation Exploration is for students to estimate a sum of two decimals. The numbers are complex to encourage students to make an estimate which means identifying that the leading digits, rather than the decimal places, are the most important for making a good estimate. 

In this lesson, students start to work with sums of larger numbers. Making a mental estimate before calculating is a valuable skill to help confirm the reasonableness of a solution.

Launch

  • Groups of 2
  • Display the expression.
  • “What is an estimate that’s too high? Too low? About right?”
  • 1 minute: quiet think time
Teacher Instructions
  • “Discuss your thinking with your partner.”
  • 1 minute: partner discussion
  • Record responses.

Student Task

1,987.89+658.541,987.89 + 658.54

Record an estimate that is:

too low about right too high

Sample Response

Sample responses:

  • Too low: 2,000 to 2,600
  • About right: 2,640 to 2,650
  • Too high: 2,658 to 2,659
Activity Synthesis (Teacher Notes)
  • “Which digits in the numbers were most important for making your estimate?” (The 1 and first 9 from the first number and the 6 and 5 from the second number.)
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.

20 min

15 min