Math lesson plan · 2nd grade (Year 3 in England) · 45 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 · 5 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 40 crayons in a box and 34 on the table. How many crayons are there in all? → 74Prompt: Add the tens, then add the ones.
- Leo collected 26 cookies in the morning and 36 in the afternoon. How many is that altogether? → 62Prompt: Add the tens, then add the ones.
- There are 33 shells in a box and 15 on the table. How many shells are there in all? → 48Prompt: Add the tens, then add the ones.
Hook · 5 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 · 10 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 · 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 that 24 + 21 = 45? (21 + 21 = 42 / 24 − 21 = 3 / 45 + 21 = 66 / 45 − 21 = 24) → 45 − 21 = 24Prompt: Subtraction undoes addition.
- Daniel says 94 − 64 = 30. Check by adding 30 + 64. Is Daniel right? (Yes / No) → YesPrompt: Work out 30 + 64.
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.
- Noah says 60 − 17 = 43. Check by adding 43 + 17. Is Noah right? (Yes / No) → YesPrompt: Work out 43 + 17.
- Which calculation checks that 44 + 26 = 70? (26 + 26 = 52 / 70 + 26 = 96 / 44 − 26 = 18 / 70 − 26 = 44) → 70 − 26 = 44Prompt: Subtraction undoes addition.
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