Adding To and Subtracting Down To a Decuple (Large Group)

6-A-L-1

Purpose: Develop initial mental strategies for addition and subtraction involving adding on from a decuple and subtracting down to a decuple (e.g. 40 + 3 and 43 – x = 40)

Description: Students solve addition and subtraction tasks involving a 2-digit number and a 1-digit number. Tasks are presented on an empty number line, making connections to written equations.

Materials

  • 100-bead string
  • Base-10 dot materials (variation)
Adding To and Subtracting Down To a Decuple

What To Say and Do

  • 1

    Using a 100-bead string, push over 40 beads and say, “Here are 40 beads. How many beads will we have if I push over 3 more?" Allow for students to respond and push over the 3 beads.

  • 2

    Facilitate a class discussion using prompts like:

    • How did you know?
    • What do you notice?
    • Why are there different colors?
  • 3

    Above the bead string, write the equation that corresponds with the task, 40 + 3 = 43.

  • 4

    Using the 43 beads on the bead string, ask students, “If we started at 43 and took away 3, how many beads will we have?” After students respond, write the equation below the bead string that corresponds with the task, 43 – 3 = 40.

  • 5

    Facilitate a class discussion asking students to share their noticings and reflections about the tasks.

    • What connections can you make from the addition and subtraction equations?
    • What do you notice?
    • How do the colors of the bead string help you?
    • If you counted, is there a way to solve the tasks without counting by ones?
  • 6

    Continue to present tasks in the same manner. (e.g., 70 + 4 and 74 – 4) 

  • 7

    As students demonstrate facility, present tasks that build in complexity:

    • If I have 35 beads and subtracted some to get 30, how much did I subtract?
    • If I have 50 beads and added some to get 54, how much did I add?

Variations

Use Base-10 dot materials as a visual model for students needing support in connecting to the quantity to the tasks being worked with.

As students become more facile in their mental computation, consider recording student thinking using an empty number line as a method of notation.

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