Math Background

Lesson: Adding and Subtracting Decimals
Developing the Concept

As long as students remember to align the numbers at the decimal point (or the ones place) in addition and subtraction problems, they should do as well with decimal computation as they do with whole-number computation. This game gives students a chance to generate decimal numbers, write and solve problems, and check decimal addition and subtraction.

  • Say: I'm going to divide the class into two teams. We'll call you the Greater Than Five team and the Less Than Five team.
    Seat members of each team together and have each team count off 1, 2, 3,
  • Ask: Will Player 1 for each team come to the board to be the Computer? Player 2 from the Greater Than Five team, please give me a decimal number greater than five.
    Each Player 1 should have a workspace at the board and a piece of chalk (or a marker). Both should write the number given by Player 2.
  • Ask: Player 2 from the Less Than Five team, will you give me a decimal number less than five?
    Each Player 1 should write the second number under the first, preparing to compute.
  • Say: Add.
    Teams get one point for a correct answer (don't push for fast computation yet). After players have a sum, allow their teams to consult to check the work, reserving the one-point award for the final sum. This helps shy or uncertain students feel more comfortable with the role of Computer.
  • Say: Erase the sum. Now, subtract.
    Again, teams get one point for a correct answer.
  • Ask: Will Player 2 for each team come to the board? Player 3 from the Greater Than Five team, give me a decimal number greater than five.
    Continue with the game until each member of the team has had a chance to be the Computer.

Wrap-Up and Assessment Hints
Present an exercise involving different numbers of decimal places either orally or in equation form. Ask students to do the following.

  • Write the exercise in preparation for computing.
  • Explain how they lined up the digits.
  • Compute and explain each step of the computation.
  • Write a paragraph explaining how this computation is similar to computing with whole numbers.

If students have trouble writing mathematics, you could get them started by asking them to show the work to you and tell you what they're doing. You might even want to let them make a video in which they narrate their progress through the computation. (The camera will have to aim over the student's shoulder so you can see that the explanation is clearly related to the work.)


Houghton Mifflin Math Grade 4