Mathematics · Grade 2

Active learning ideas

Finding the Missing Number

Turn your students into math detectives! This topic introduces the exciting challenge of finding the missing number in an equation.

• Ontario Curriculum Expectations
• Ontario Curriculum (2020): Grade 2 - Algebra - C2.3

• 15–20 min
• Pairs → Whole Class
• 3 activities

Activity 01

• Mystery Object
• 20 min · Pairs

Missing Number Detectives

Students use magnifying glasses and 'clue cards' (equations with a missing number) placed around the classroom. They solve for the missing number on their recording sheet, becoming 'detectives' who crack the code.

Explain the strategy you used to find the missing number in 8 + ? = 15.

Facilitation Tip

Encourage students to use different strategies and share their methods with their partner.

What to look for

Use an 'exit ticket' with two missing number problems. This provides a quick snapshot of student understanding at the end of a lesson.

• Understand
• Analyze
• Evaluate
• Self-Management
• Social Awareness

Activity 02

• Mystery Object
• 15 min · Small Groups

Balance the Scale

Using a pan balance, place cubes representing the total on one side (e.g., 15) and the known part on the other (e.g., 8). Students add cubes to the second side until the scale balances, discovering the missing number.

Compare solving for a missing number in an addition equation versus a subtraction equation.

Facilitation Tip

This activity provides a powerful visual representation of the equals sign meaning 'the same as'.

What to look for

Observe students as they work in pairs with manipulatives. Listen to their conversations to assess their reasoning and use of mathematical language.

• Understand
• Analyze
• Evaluate
• Self-Management
• Social Awareness

Activity 03

• Mystery Object
• 15 min · Whole Class

Number Line Hops

On a large floor number line, a student starts at the known addend (e.g., 8 in 8 + ? = 15). They then 'hop' to the sum (15), counting the number of hops to find the missing part.

Identify the unknown value in a given equation.

Facilitation Tip

Ask students to demonstrate both 'hopping on' for addition and 'hopping back' for subtraction.

What to look for

Administer a short quiz that includes a variety of equation structures, including addition and subtraction with the unknown in all possible positions.

• Understand
• Analyze
• Evaluate
• Self-Management
• Social Awareness

A few notes on teaching this unit

Begin with concrete materials like counters to model the 'part-part-whole' relationship in an equation. Guide students to see the connection between addition and subtraction, using fact families as a tool. Gradually remove the scaffolds, moving from manipulatives to drawings, and finally to solving the abstract equations.

Students will learn to solve for an unknown in addition and subtraction equations and will be able to explain the strategies they used.

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Watch Out for These Misconceptions

• Students automatically add the two numbers they see in an equation, regardless of the unknown's position. For example, in 7 + ? = 12, they might calculate 7 + 12 = 19.

  Reinforce that the equals sign means 'balance' or 'the same as'. Use a pan balance or a part-part-whole mat to show that the two parts must combine to make the whole, not a larger number.

• Students believe the unknown or the box must always be the answer and come after the equals sign.

  Present equations in various formats, such as ? + 5 = 11 or 16 = ? + 9. Explain that the box is just a placeholder for a number we need to find to make the equation true.

• When solving a subtraction problem like 14 – ? = 6, students subtract the smaller number from the larger one (14 – 6) without understanding why it works.

  Connect this to the inverse operation. Explain that if 14 minus something equals 6, then 6 plus that same something must equal 14. Use fact families to demonstrate this relationship visually.

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Methods used in this brief

• Mystery Object