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Parent guide · Grade 2 Math · Unit 4

Lesson 4.2 Regrouping a Ten

Written for the parent. Read the Say this lines aloud as they are, or in your own words. Answers for you are marked Listen for.

At a glance

Time
5 sessions of 20–25 minutes
Goal
Regroup 1 ten as 10 ones to subtract.
You need
  • Dice
  • Pencil and crayons
Print
Worksheet A · Worksheet B · the lesson page

Steps

  1. Do this: Ask these out loud. Your child answers out loud; no paper needed.

    • Ask: “40 − 10” Listen for: 30
    • Ask: “98 − 90” Listen for: 8
    • Ask: “97 − 40” Listen for: 57
    • Ask: “46 − 30” Listen for: 16
    • Ask: “40 − 20” Listen for: 20

  2. Say this: “52 − 27: you cannot take 7 from 2, so regroup a ten. 5 tens 2 ones becomes 4 tens 12 ones.”

    Say this: “12 − 7 = 5 and 4 − 2 = 2, so the answer is 25.”

    Say this: “Regroup: trade 1 ten for 10 ones.”

  3. Show: 41 − 22 = ___

    Say this: “Ones: 1 is less than 2, so regroup 1 ten as 10 ones. The tens digit becomes 3.”

    Say this: “11 − 2 = 9”

    Say this: “Tens: 3 − 2 = 1”

    Say this: “Check: 19 + 22 = 41”

    Answer: 19

    Show: A blacksmith had 93 nails. He used 77 to shoe the horses. How many nails are left?

    bar model

    Say this: “Unknown: how many are left.”

    Say this: “Bar model: the whole is 93; one part is 77; the other part is unknown.”

    Say this: “Equation: 93 − 77 = ?”

    Say this: “93 − 77 = 16”

    Say this: “Answer: 16 nails are left.”

    Answer: 16 nails. 16 nails are left.

  4. Do this: Your child does these with you. Let them try first; help only after a real try.

    Ask: 32 − 26 = ___

    Listen for: 6

    If stuck: Ones: 2 is less than 6, so regroup 1 ten as 10 ones. The tens digit becomes 2.

    Ask: 33 − 14 = ___

    Listen for: 19

    If stuck: Ones: 3 is less than 4, so regroup 1 ten as 10 ones. The tens digit becomes 2.

    Ask: 70 − 65 = ___

    Listen for: 5

    If stuck: Ones: 0 is less than 5, so regroup 1 ten as 10 ones. The tens digit becomes 6.

    7 more practice items on the lesson page, and on Worksheet A and Worksheet B.

    • Subtracting the smaller digit from the bigger: 7 − 2 = 5 in the ones.

  6. Start at 99. Roll two dice, make a two-digit number and subtract it. Check your partner’s answer by adding. First to go below 10 wins.

  7. Say this: “Tell me what you learned today. Show me with your hands or the counters if you like.”

    Listen for: Regroup 1 ten as 10 ones to subtract.

  8. One-minute subtraction drill on Fridays. Mental math: take 10 or 20 away from any two-digit number.

Spread over the week

5 sessions of 20–25 minutes. Stop while it is still going well.

  1. Session 1. Warm-up, then teach the new idea (steps 1 and 2).
  2. Session 2. Warm-up again, then show how with the examples.
  3. Session 3. Practice together, then play the game.
  4. Session 4. More practice on the worksheet, and the daily habit.
  5. Session 5. Quick check: narration and a few practice items. Move on when it is mostly right.