Math lesson plan · 3rd grade (Year 4 in England) · 45 minutes
Check answers with the inverse operation
Addition and subtraction
CCSS 3.NBT.A.2England: Y3 Addition and subtractionEngland: Y4 Addition and subtraction
Objective and success criteria
Students will: Check answers with the inverse operation.
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Add three-digit numbers with regrouping; Subtract three-digit numbers with exchanging
Leads to: the next skill in this area.
Materials: Base-ten blocks, Number lines, Hundred square, Mini whiteboards and pens, Printed student sheet (below).
Warm-up · 5 min
- Quick retrieval from earlier learning (Add three-digit numbers with regrouping; Subtract three-digit numbers with exchanging). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- 305 + 230 = ? → 535Prompt: Add the ones first.
- 608 − 478 = ? → 130Prompt: Start with the ones.
- Ana's school collected 495 cans for recycling in March and 229 in April. How many cans is that altogether? → 724Prompt: Altogether means add.
Hook · 5 min
- Give a real shopping total or a score difference and ask for a quick estimate first.
- Share the objective: Check answers with the inverse operation. Read the success criteria together.
I do: model it · 10 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: Which calculation checks 570 − 102 = 468?
- Step 1: 468 + 102 = 570
- Step 2: 468 + 102
- Answer: 468 + 102
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 10 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- Which calculation checks 338 − 262 = 76? (338 + 262 / 338 − 76 − 262 / 76 + 262 / 76 − 262) → 76 + 262Prompt: Addition undoes subtraction.
- Mateo says 613 + 243 = 856. Mateo checks with 856 − 243. Is Mateo's answer correct? (yes / no) → yesPrompt: Work out 856 − 243.
You do: independent practice · 10 min
- Students work on the student sheet. Circulate and check the first two answers of every student early.
- Students who finish: write a question of their own for a partner, with the answer.
The 8 questions are on the student sheet below, with answers in the key.
Exit ticket · 5 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- Ana says 399 + 104 = 493. Ana checks with 493 − 104. Is Ana's answer correct? (yes / no) → noPrompt: Work out 493 − 104.
- Which calculation checks 685 − 513 = 172? (685 − 172 − 513 / 685 + 513 / 172 + 513 / 172 − 513) → 172 + 513Prompt: Addition undoes subtraction.
Watch out for
- Subtracting the smaller digit from the bigger one in every column (52 − 17 = 45).
- Forgetting to add the regrouped (carried) ten.
Support and stretch
Support
- Review first: Add three-digit numbers with regrouping.
- Use concrete materials: base-ten blocks.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/math/grade-3.
Free lesson plan from eurekami.com/lesson-plan-builder