Mathematics · Grade 2

Active learning ideas

Mental Math: Adding and Subtracting 10 or 100

Active learning helps students internalize place value changes when adding or subtracting 10 or 100. Movement and hands-on tasks make abstract digit shifts concrete, especially for learners who benefit from visual and kinesthetic input. These activities turn number patterns into memorable experiences.

• Ontario Curriculum Expectations
• 2.NBT.B.8

• 15–30 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Stations Rotation
• 25 min · Whole Class

Number Line Hops: Place Value Jumps

Mark a floor number line from 100 to 900 with tape. Call a starting number and operation, such as 'Begin at 250, subtract 100.' Students hop to the landing spot and call out the result. Switch roles so different students lead calls.

What happens to the tens digit when you add 10 to a number?

Facilitation Tip

During Number Line Hops, have students physically step forward or back to reinforce the size of the 10 or 100 jump.

What to look for

Present students with a number, for example, 452. Ask them to write down the result of adding 10, subtracting 10, adding 100, and subtracting 100. Observe their written answers for accuracy.

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Activity 02

• Stations Rotation
• 30 min · Small Groups

Base-10 Block Builds: Small Groups

Supply place value mats and base-10 blocks. Groups construct a three-digit number, then mentally add or subtract 10 or 100 before rebuilding to verify. Chart the digit changes observed. Share one group strategy with the class.

How can you use place value to add or subtract 100 from a number up to 200?

Facilitation Tip

In Base-10 Block Builds, ask students to verbalize each move, such as 'I added a ten rod, so the tens digit increased by one.'

What to look for

Pose the question: 'What happens to the number 789 when you subtract 10? What happens when you subtract 100? Explain your thinking using place value.' Listen for students' explanations of how the tens and hundreds digits change.

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Activity 03

• Stations Rotation
• 20 min · Pairs

Partner Strategy Duels: Pairs

Pairs use whiteboards and face off. Teacher announces a number and operation; both solve mentally and reveal answers. The quicker correct pair explains their thinking. Rotate partners after five rounds.

Can you show what happens when you subtract 10 from 150?

Facilitation Tip

For Partner Strategy Duels, set a timer so both students explain their method within the same time limit.

What to look for

Give each student a card with a number (e.g., 315). Ask them to write one sentence describing what happens to the number when you add 10, and another sentence describing what happens when you add 100. Collect the cards to review understanding.

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Activity 04

• Stations Rotation
• 15 min · Individual

Benchmark Flash Cards: Individual

Distribute cards with numbers 100-900. Students flip cards and mentally compute +10, -10, +100, or -100, recording answers. Time themselves for fluency growth. Pair up to check and discuss errors.

What happens to the tens digit when you add 10 to a number?

What to look for

Present students with a number, for example, 452. Ask them to write down the result of adding 10, subtracting 10, adding 100, and subtracting 100. Observe their written answers for accuracy.

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

A few notes on teaching this unit

Start with Base-10 blocks to ground the concept in physical units. Use Number Line Hops to connect movement to the abstract idea of 10 or 100. Avoid rushing to algorithms; let students articulate patterns first. Research shows that verbalizing place value changes strengthens long-term retention and transfer.

Students will confidently adjust the tens or hundreds digit when adding or subtracting 10 or 100. They will explain their reasoning using place value language and correct peers’ misunderstandings with evidence from their models or number lines. Accuracy will be visible in their written work and oral justifications.

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Watch Out for These Misconceptions

• During Base-10 Block Builds, watch for students who incorrectly add or remove ones blocks instead of ten rods when adding or subtracting 10.

  Pause the activity and ask the student to count the blocks aloud, emphasizing that 10 ones make one ten rod. Have them rebuild the number correctly and restate the place value change.

• During Number Line Hops, watch for students who count by ones instead of recognizing the full 10-unit or 100-unit jump.

  Model a single hop yourself, saying, 'This jump covers exactly 10 spaces, so the tens digit will increase by one.' Invite the student to repeat the hop while counting aloud in tens.

• During Partner Strategy Duels, watch for students who claim 100 minus 100 equals zero because they only see the hundreds digit disappearing.

  Hand them a place value chart and ask them to write 100, subtract 100, and describe what is left in each column. Guide them to see that all digits become zero.

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Methods used in this brief

• Stations Rotation