Math lesson plan · 3rd grade (Year 4 in England) · 30 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 · 4 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.
- 378 + 115 = ? → 493Prompt: Add the ones first.
- 369 − 137 = ? → 232Prompt: Start with the ones.
- Kwame's school collected 229 cans for recycling in March and 439 in April. How many cans is that altogether? → 668Prompt: Altogether means add.
Hook · 3 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 · 7 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 · 6 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 895 − 153 = 742? (742 + 153 / 742 − 153 / 895 + 153 / 895 − 742 − 153) → 742 + 153Prompt: Addition undoes subtraction.
- Omar says 231 + 141 = 372. Omar checks with 372 − 141. Is Omar's answer correct? (yes / no) → yesPrompt: Work out 372 − 141.
You do: independent practice · 7 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 6 questions are on the student sheet below, with answers in the key.
Exit ticket · 3 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- Kwame says 561 + 177 = 838. Kwame checks with 838 − 177. Is Kwame's answer correct? (yes / no) → noPrompt: Work out 838 − 177.
- Which calculation checks 549 − 269 = 280? (280 − 269 / 280 + 269 / 549 − 280 − 269 / 549 + 269) → 280 + 269Prompt: 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