Math lesson plan · 3rd grade (Year 4 in England) · 45 minutes
Mental addition and subtraction by adjusting (99, 199, 101…)
Addition and subtraction
CCSS 3.NBT.A.2England: Y3 Addition and subtraction
Objective and success criteria
Students will: Mental addition and subtraction by adjusting (99, 199, 101…).
- 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 and subtract within 1000
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 and subtract within 1000). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- What is 100 less than 212? → 112Prompt: Which digit changes when you subtract 100?
- 768 − 508 = ? → 260Prompt: Line up hundreds, tens and ones.
- What is 100 less than 713? → 613Prompt: Which digit changes when you subtract 100?
Hook · 5 min
- Give a real shopping total or a score difference and ask for a quick estimate first.
- Share the objective: Mental addition and subtraction by adjusting (99, 199, 101…). 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: 388 − 101 = ?
- Step 1: 388 − 100 = 288
- Step 2: 288 − 1 = 287
- Answer: 287
- 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.
- 618 + 99 = ? → 717Prompt: 99 is close to 100.
- Tariq works out 642 − 99 by doing 642 − 100 = 542, then subtracting 1 more to get 541. What should Tariq have done, and what is 642 − 99? → 543Prompt: Taking away 100 takes away 1 too many.
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.
- Chloe works out 554 − 99 by doing 554 − 100 = 454, then subtracting 1 more to get 453. What should Chloe have done, and what is 554 − 99? → 455Prompt: Taking away 100 takes away 1 too many.
- 398 + 98 = ? → 496Prompt: 98 is close to 100.
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 and subtract within 1000.
- 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