Math lesson plan · 2nd grade (Year 3 in England) · 60 minutes
Check an answer with the inverse operation
Addition and subtraction
CCSS 2.NBT.B.9England: Y2 Addition and subtractionEngland: Y3 Addition and subtraction
Objective and success criteria
Students will: Check an answer 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 two-digit numbers with regrouping; Subtract two-digit numbers with regrouping
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 · 6 min
- Quick retrieval from earlier learning (Add two-digit numbers with regrouping; Subtract two-digit numbers with regrouping). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- There are 20 stars in a box and 58 on the table. How many stars are there in all? → 78Prompt: Add the tens, then add the ones.
- Elena collected 49 cards in the morning and 16 in the afternoon. How many is that altogether? → 65Prompt: Add the tens, then add the ones.
- Isla has 46 crayons. A friend gives Isla 26 more. How many crayons does Isla have now? → 72Prompt: Add the tens, then add the ones.
Hook · 6 min
- Give a real shopping total or a score difference and ask for a quick estimate first.
- Share the objective: Check an answer with the inverse operation. Read the success criteria together.
I do: model it · 12 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: Which calculation checks that 45 + 30 = 75?
- Step 1: 75 − 30 = 45
- Answer: 75 − 30 = 45
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 12 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 that 47 + 19 = 66? (47 − 19 = 28 / 19 + 19 = 38 / 66 − 19 = 47 / 66 + 19 = 85) → 66 − 19 = 47Prompt: Subtraction undoes addition.
- Kwame says 69 − 24 = 46. Check by adding 46 + 24. Is Kwame right? (Yes / No) → NoPrompt: Work out 46 + 24.
You do: independent practice · 17 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 · 7 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- Which calculation checks that 55 + 23 = 78? (55 − 23 = 32 / 78 + 23 = 101 / 78 − 23 = 55 / 23 + 23 = 46) → 78 − 23 = 55Prompt: Subtraction undoes addition.
- Noah says 54 − 33 = 21. Check by adding 21 + 33. Is Noah right? (Yes / No) → YesPrompt: Work out 21 + 33.
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 two-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-2.
Free lesson plan from eurekami.com/lesson-plan-builder