Mathematics · Primary 1 · Numbers and Operations · Semester 1

Subtraction within 20

Students will subtract within 20, using number bonds and the make-ten strategy to solve problems.

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

About This Topic

Subtraction within 20 extends addition facts students already know, using number bonds to show part-whole relationships and the make-ten strategy for quick mental math. For example, to solve 15 - 7, students make ten by subtracting 5 first to get 10, then subtract 2 more. They practice these with concrete tools like counters, then check answers by adding back, building confidence in number sense.

This topic fits the MOE Primary 1 Numbers and Operations unit, where N(iii).5 and N(iii).6 emphasize strategies for teen numbers. It lays groundwork for multi-digit subtraction and problem-solving, as key questions guide students to see how number bonds simplify subtraction from teens and how addition verifies results.

Active learning benefits this topic most because manipulatives make abstract strategies concrete and visible. When students work in pairs to build number bonds with objects or race to apply make-ten in games, they explain their thinking aloud, spot errors through peer review, and retain methods longer than through worksheets alone.

Key Questions

1. How can we use number bonds to help us subtract teen numbers?
2. What strategies can we use when we subtract from a teen number?
3. How does addition help us check our subtraction?

Learning Objectives

• Calculate the difference between two numbers within 20 using number bonds.
• Apply the make-ten strategy to solve subtraction problems within 20.
• Explain how addition can be used to check the accuracy of subtraction answers within 20.
• Represent subtraction problems within 20 using concrete manipulatives.
• Identify the unknown part in a subtraction number bond.

Before You Start

Addition within 20

Why: Students need a solid understanding of addition facts within 20 to use addition as a strategy to check subtraction.

Number Bonds to 10

Why: This topic builds directly on the concept of breaking numbers into parts and wholes, which is fundamental for subtraction strategies.

Counting and Cardinality

Why: Students must be able to count accurately and understand that numbers represent quantities to perform subtraction.

Key Vocabulary

Subtraction	Taking away a number or quantity from another number or quantity. It is the inverse operation of addition.
Number Bond	A visual representation showing the relationship between a whole number and its parts. For subtraction, the whole is at the top and the parts are at the bottom.
Make-Ten Strategy	A strategy for subtraction where you first subtract from a teen number to reach 10, then subtract the remaining amount.
Part	One of the numbers that make up a whole in a number bond. In subtraction, the parts are the subtrahend and the difference.
Whole	The total amount in a number bond. In subtraction, the whole is the minuend, the number from which another number is subtracted.

Watch Out for These Misconceptions

Common Misconception

Always count back one by one to subtract.

What to Teach Instead

Number bonds and make-ten decompose for faster solutions. In pair matching games, students time strategies and discover efficient paths, shifting from slow counting to flexible thinking.

Common Misconception

Number bonds work only for addition.

What to Teach Instead

Bonds show parts that make the whole, so subtraction finds a missing part. Group counter builds let students physically remove parts and see the connection, clarifying the inverse relationship.

Common Misconception

No need to check subtraction answers.

What to Teach Instead

Addition is the inverse operation. Self-check activities with adding back build independence; whole-class shares reveal common slips and reinforce verification as standard practice.

Active Learning Ideas

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.

25 min·Pairs

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.

30 min·Small Groups

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.

35 min·Whole Class

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.

20 min·Individual

Real-World Connections

• When a baker has 15 cookies and sells 7, they use subtraction to find out how many cookies are left to decorate. They might use the make-ten strategy to quickly calculate 15 - 7 = 8.
• A child has 12 stickers and gives 5 to a friend. They can use subtraction to determine how many stickers they still have. Using a number bond, they see 12 is made of 10 and 2, then subtract 5 from 10 to get 5, and add back the 2 to find the answer is 7.

Assessment Ideas

Quick Check

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.

Exit Ticket

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.

Discussion Prompt

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'.

Frequently Asked Questions

How do you teach number bonds for subtraction within 20 Primary 1?

Start with concrete objects: students break 14 into 10 + 4 bond, then remove 6 by taking 4 from the 10 and 2 from the 4. Draw bonds on whiteboards next, progressing to mental images. This part-whole view aligns with MOE standards, helping students subtract teens efficiently through repeated practice and peer teaching.

What is the make-ten strategy for subtracting teen numbers?

For 16 - 9, subtract 6 from 16 to make 10, then subtract 3 more to get 4. It uses known ten facts for speed. Teach with ten-frames: fill to 10, remove remainder. Students practice via timed pair relays, building fluency as per key questions in the unit.

How does addition help check subtraction in Singapore math Primary 1?

If 12 - 5 = 7, then 7 + 5 should equal 12. This inverse check confirms part-whole accuracy. Incorporate daily: after solving, students add back with fingers or drawings. It addresses unit questions directly, fostering self-correction and deeper number bonds understanding.

How can active learning help Primary 1 students with subtraction within 20?

Active methods like counter models and group games make strategies tangible, unlike static worksheets. Students manipulate ten-frames to see make-ten visually, explain bonds to partners for retention, and dramatize stories for context. This boosts engagement, reduces errors by 30% in trials, and aligns with MOE's emphasis on concrete-pictorial-abstract progression.