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Parent guide · Grade 2 Math · Unit 5

Lesson 5.4 Mental Math with 10 and 100

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
Add and subtract 10 and 100 in your head.
You need
  • 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: “What number is ten more than 832?” Listen for: 842
    • Ask: “What number is ten more than 270?” Listen for: 280
    • Ask: “What number is ten more than 439?” Listen for: 449
    • Ask: “What number is ten more than 366?” Listen for: 376
    • Ask: “What number is ten less than 605?” Listen for: 595

  2. Say this: “Adding 10 changes the tens digit by one; adding 100 changes the hundreds digit by one.”

    Say this: “Add 99 by adding 100 and taking 1 away.”

    Say this: “Mental math: working a problem in your head.”

  3. Show: What number is one hundred less than 703?

    Say this: “703 − 100 = 603”

    Answer: 603

    Show: 117 + 61 = ___

    Say this: “Add the tens: 117 + 60 = 177”

    Say this: “Add the ones: 177 + 1 = 178”

    Answer: 178

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

    Ask: What number is ten less than 914?

    Listen for: 904

    If stuck: 914 − 10 = 904

    Ask: What number is ten more than 162?

    Listen for: 172

    If stuck: 162 + 10 = 172

    Ask: What number is ten more than 602?

    Listen for: 612

    If stuck: 602 + 10 = 612

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

    • Changing the ones digit.

  6. Price a few household items between 100 and 500 cents. Your child adds two prices on paper and checks with a partner. Then find how much more one costs than the other.

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

    Listen for: Add and subtract 10 and 100 in your head.

  8. Count by tens and hundreds from any number, forward and back.

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.