Mental Addition and Subtraction StrategiesActivities & Teaching Strategies
Active learning helps Primary 2 students internalize mental addition and subtraction strategies by giving them repeated, low-stakes practice in real time. When students articulate their thinking aloud to peers, they notice multiple pathways and build confidence in flexible computation.
Learning Objectives
- 1Calculate the sum or difference of two 2-digit numbers using the making tens strategy.
- 2Apply the compensation strategy to mentally add or subtract 2-digit numbers efficiently.
- 3Explain how to use known addition facts to derive answers for subtraction problems involving 2- and 3-digit numbers.
- 4Compare the efficiency of different mental addition and subtraction strategies for a given problem.
- 5Solve word problems involving 2- and 3-digit addition and subtraction using mental strategies.
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Pairs: Strategy Swap Game
Partners draw problem cards like 56 + 37. One solves mentally, explains the strategy used, such as making tens. Switch roles after three problems and compare methods. End with partners creating their own problem.
Prepare & details
How does making a ten or hundred make addition easier?
Facilitation Tip: During Strategy Swap Game, circulate and listen for pairs to name the strategy they just used, not just the answer.
Setup: Standard classroom seating; students turn to a neighbor
Materials: Discussion prompt (projected or printed), Optional: recording sheet for pairs
Small Groups: Compensation Hunt
Provide cards with addition problems suited for compensation. Groups sort them into 'easy to compensate' and 'other strategies,' solve each, and justify choices. Share one group example with the class.
Prepare & details
What is the compensation strategy, and when is it useful?
Facilitation Tip: During Compensation Hunt, provide mini whiteboards so students can record both the adjusted and final numbers.
Setup: Standard classroom seating; students turn to a neighbor
Materials: Discussion prompt (projected or printed), Optional: recording sheet for pairs
Whole Class: Mental Math Number Talk
Pose problems like 81 - 29 on the board. Students use signals to share strategies silently, then volunteers explain aloud. Record multiple methods on chart paper for reference.
Prepare & details
How can we use known addition facts to solve subtraction problems mentally?
Facilitation Tip: During Mental Math Number Talk, record all student ideas on the board, even if some are less efficient, to show thinking diversity.
Setup: Standard classroom seating; students turn to a neighbor
Materials: Discussion prompt (projected or printed), Optional: recording sheet for pairs
Individual: Strategy Journal Reflection
Students solve five mixed problems mentally, note the strategy chosen and why. Draw quick sketches if needed, like tens frames. Share one entry with a partner.
Prepare & details
How does making a ten or hundred make addition easier?
Setup: Standard classroom seating; students turn to a neighbor
Materials: Discussion prompt (projected or printed), Optional: recording sheet for pairs
Teaching This Topic
Start with the simplest strategy, making tens, because it reduces cognitive load. Avoid teaching written-column language like ‘carry’ or ‘borrow’ as it can anchor students to paper methods. Research shows that students who verbalize their steps develop stronger number sense than those who only compute silently.
What to Expect
Students will select and use at least two different mental strategies for each problem and explain their choice to a partner. They will demonstrate speed and accuracy without relying on written steps or counting on fingers.
These activities are a starting point. A full mission is the experience.
- Complete facilitation script with teacher dialogue
- Printable student materials, ready for class
- Differentiation strategies for every learner
Watch Out for These Misconceptions
Common MisconceptionDuring Strategy Swap Game, watch for students who always add ones first instead of making tens.
What to Teach Instead
Prompt pairs to test both ways and time themselves to see which method finishes first, then discuss why making tens is faster.
Common MisconceptionDuring Compensation Hunt, watch for students who only adjust numbers that end in 9 or 1.
What to Teach Instead
Ask groups to adjust their sorting rules and find three examples where they add or subtract 2, 3, or other numbers to build broader understanding.
Common MisconceptionDuring Mental Math Number Talk, watch for students who immediately count back when subtracting.
What to Teach Instead
Highlight the connection between subtraction and addition by asking, 'What do you already know that can help you solve this?' and recording their related addition facts on the board.
Assessment Ideas
After Strategy Swap Game, present a series of problems and ask students to write the strategy they used and their answer on a sticky note before placing it on the board under the correct strategy label.
During Mental Math Number Talk, pose a context problem and ask students to share two different strategies. Listen for mentions of compensation, making tens, or known facts to assess strategy flexibility.
After Compensation Hunt, give each student a card with a problem like '45 - 18'. Ask them to write the answer and a one-sentence explanation of the strategy they chose, such as 'I made 18 into 20 and adjusted by adding 2 back.'
Extensions & Scaffolding
- Challenge: Ask students to find two different strategies for the same problem and compare which felt faster or easier.
- Scaffolding: Provide number lines or ten-frame cards for students to visualize partitioning before they compute.
- Deeper exploration: Introduce problems with three addends, such as 24 + 37 + 16, to extend compensation and grouping strategies.
Key Vocabulary
| Making Tens | A strategy where you adjust one number to make a multiple of ten, then add or subtract the remaining part. For example, 47 + 5 becomes 47 + 3 + 2. |
| Compensation | A strategy where you adjust one number to make it easier to calculate, then adjust the answer to compensate for the change. For example, 65 + 49 becomes 65 + 50 - 1. |
| Known Facts | Addition or subtraction facts that students have memorized or can quickly recall, such as doubles (e.g., 7 + 7) or facts that make ten (e.g., 6 + 4). |
| Mental Math | Performing calculations in your head without using written methods or a calculator. |
Suggested Methodologies
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5E Model
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