Mathematics · Primary 1

Active learning ideas

Subtraction within 20

Active learning transforms subtraction within 20 from abstract symbols into concrete understanding. When students manipulate objects, partner with peers, and dramatize situations, they see how numbers break apart and recombine, turning rote counting into flexible thinking. These hands-on experiences build the mental models that make mental math possible.

• MOE Syllabus Outcomes
• MOE: N(iii).5
• MOE: N(iii).6

• 20–35 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Think-Pair-Share
• 25 min · Pairs

Pair Work: Number Bond Subtraction Cards

Prepare cards with subtraction problems within 20 and blank number bonds. Pairs draw a problem, draw the bond to decompose, solve using make-ten if needed, and check by adding. Switch cards every 3 minutes and discuss strategies.

How can we use number bonds to help us subtract teen numbers?

Facilitation Tip

During Pair Work: Number Bond Subtraction Cards, circulate and ask each pair to verbalize their number bond before solving the subtraction problem, ensuring they connect the parts to the whole.

What to look for

Present students with a number bond template for 13. Ask them to fill in the parts (e.g., 10 and 3). Then, pose the subtraction problem 13 - 4. Have students use their number bond to solve it, showing their steps.

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Activity 02

• Think-Pair-Share
• 30 min · Small Groups

Small Groups: Make-Ten Counter Challenge

Give each group counters and ten-frames. One student poses a subtraction like 17 - 8; others model making ten on the frame, subtract the rest, and verify with addition. Rotate roles after two problems.

What strategies can we use when we subtract from a teen number?

Facilitation Tip

During Small Groups: Make-Ten Counter Challenge, watch that students physically move counters into groups of ten before subtracting the remaining amount, reinforcing the decomposition process.

What to look for

Give each student a card with a subtraction problem, such as 16 - 9. Ask them to write the number bond for 16, solve the problem using the make-ten strategy, and then write an addition sentence to check their answer.

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Activity 03

• Think-Pair-Share
• 35 min · Whole Class

Whole Class: Subtraction Story Dramatization

Read a story problem aloud, such as '18 apples, eat 9'. Class uses personal counters to act out number bonds and make-ten steps on desks, then shares answers chorally and checks with addition.

How does addition help us check our subtraction?

Facilitation Tip

During Whole Class: Subtraction Story Dramatization, invite students to act out the story while you record their actions as equations on the board, linking movement to symbolic representation.

What to look for

Ask students: 'Imagine you have 11 apples and you eat 3. How can you use the make-ten strategy to figure out how many apples are left? Explain your steps.' Listen for their use of 'take apart 11 into 10 and 1' and 'subtract 3 from 10'.

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Activity 04

• Think-Pair-Share
• 20 min · Individual

Individual: Strategy Choice Boards

Provide boards with 8 problems within 20. Students choose number bonds or make-ten for each, draw their work, and add back to check. Circulate to prompt explanations.

How can we use number bonds to help us subtract teen numbers?

What to look for

Present students with a number bond template for 13. Ask them to fill in the parts (e.g., 10 and 3). Then, pose the subtraction problem 13 - 4. Have students use their number bond to solve it, showing their steps.

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

A few notes on teaching this unit

Teachers approach subtraction within 20 by first anchoring the work in concrete tools, then scaffolding toward mental strategies. Avoid rushing to abstract symbols before students can explain their moves with objects or drawings. Research shows that students who can decompose numbers flexibly in first grade perform better in later math, so prioritize verbal explanations over speed. Use consistent language like ‘make ten’ and ‘take apart’ to build a shared mental model across activities.

Success looks like students choosing the make-ten strategy over counting one-by-one, explaining their steps with number bonds, and verifying answers independently. You will notice students moving from slow, single-count strategies to efficient decompositions, and explaining their work with phrases like ‘take apart 11 into 10 and 1’ without prompting.

---

Watch Out for These Misconceptions

• During Pair Work: Number Bond Subtraction Cards, watch for students who skip the number bond step and count back one-by-one from the minuend.

  Prompt the pair to build the minuend with counters and circle the group of ten before removing any. Ask them to name the parts aloud before writing the equation, reinforcing the bond structure.

• During Small Groups: Make-Ten Counter Challenge, watch for students who remove counters without grouping ten first, treating the counters as single units.

  Model splitting the minuend into ten and the rest, then ask the group to agree on the ten-group before removing any counters. Have them say the split aloud as they build it.

• During Whole Class: Subtraction Story Dramatization, watch for students who act out the story but write the equation incorrectly, mixing up minuend and subtrahend.

  After the dramatization, ask the class to identify the starting amount and the amount removed before recording. Use gestures to connect the action to the equation.

---

Methods used in this brief

• Think-Pair-Share