Parent guide · Grade 3 Math · Unit 2

Lesson 2.3 Mental Math Strategies

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 25–30 minutes
Goal: Add and subtract in your head using tens and compensation.
You need: Pencil and crayons
Print: Worksheet A · Worksheet B · the lesson page

Steps

Warm-up
Do this: Ask these out loud. Your child answers out loud; no paper needed.
- Ask: “400 + 200” Listen for: 600
- Ask: “300 + 200” Listen for: 500
- Ask: “600 + 600” Listen for: 1,200
- Ask: “500 + 100” Listen for: 600
- Ask: “600 + 700” Listen for: 1,300
Teach the new idea
Say this: “To add 199, add 200 and take 1 away.”
Say this: “To subtract 98, subtract 100 and add 2 back.”
Say this: “Compensation: change a number to make it easy, then fix the answer.”
Show how
Show: 68 + 253 = ___
Say this: “Add the tens: 68 + 250 = 318”
Say this: “Add the ones: 318 + 3 = 321”
Answer: 321
Show: 785 − 86 = ___
Say this: “Take away the tens: 785 − 80 = 705”
Say this: “Take away the ones: 705 − 6 = 699”
Answer: 699
Practice together
Do this: Your child does these with you. Let them try first; help only after a real try.
Ask: 600 + 100 = ___
Listen for: 700
If stuck: Add the tens: 600 + 100 = 700
Ask: 300 + 700 = ___
Listen for: 1,000
If stuck: Add the tens: 300 + 700 = 1,000
Ask: 400 + 400 = ___
Listen for: 800
If stuck: Add the tens: 400 + 400 = 800
7 more practice items on the lesson page, and on Worksheet A and Worksheet B.
Watch for
- Fixing the answer in the wrong direction.
Game: Cross Check
One player adds two 3-digit numbers on paper while the other estimates. Compare: is the exact answer close to the estimate? Swap roles.
Narration
Say this: “Tell me what you learned today. Show me with your hands or the counters if you like.”
Listen for: Add and subtract in your head using tens and compensation.
Daily habit
Mental math warm-up: add or subtract 10, 100 or 1,000.

Spread over the week

5 sessions of 25–30 minutes. Stop while it is still going well.

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