Primary 1 Maths | Subtraction within 20 Lesson-Prep Guide (for tutors)

Primary 1 Maths | Subtraction within 20 Lesson-Prep Guide (for tutors) | Take-away, comparison, checking

2026-11-08

Tutor lesson-prep guides · Primary Maths

Subtraction carries two different meanings: taking away, and finding the difference. A pupil who has only met take-away will force the wrong model onto “Amy has 12, Ben has 7, how many more does Amy have?”.

Objectives — The pupil can: (1) explain and carry out subtraction within 20 as take-away; (2) answer difference questions; (3) check subtraction with addition (7 + 5 = 12, so 12 − 7 = 5); (4) tell which type a question is.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Number bonds to 10	instant recall of 1+9 to 5+5	needed for make-ten in reverse
Addition within 20	can use make-ten	needed for the check

02 Lesson flow (40 minutes)

Time	Stage	Tutor does	Student does
0–5 min	Diagnose	12 counters, take away 7	answers 5
5–15 min	Take-away	12 − 7: subtract down to 10 first	writes 12 − 2 − 5 = 5
15–25 min	Comparison	12 against 7, how many more	same expression, new meaning
25–33 min	Check with addition	7 + 5 = 12	so 12 − 7 = 5
33–40 min	Exit check	three questions	all correct

03 Teaching script (read it out)

Scene 1: Act 1: Take-away, subtracting down to ten

For 12 − 7, do not count back seven places. Subtract 2 to reach 10, then subtract the remaining 5, giving 5. Write 12 − 2 − 5 = 5. This mirrors make-ten in addition: 10 is the halfway station. The pupil should get used to reaching 10 first and then dealing with the rest.

Tutor line: “12 minus 2 is 10. How much of the 7 is left to subtract? Five. So what is 10 minus 5?”

Scene 2: Act 2: Comparison, how many more

Lay 12 red and 7 blue counters in two aligned rows. Ask how many more red than blue. The answer is the extra 5 after lining up. This uses the same expression as take-away but a different meaning: finding the excess rather than removing.

Tutor line: “Line the rows up. How many extra red are sticking out? That is the difference.”

Scene 3: Act 3: Check with addition

Teach that subtraction can be checked by addition: 7 + 5 = 12, so 12 − 7 should be 5. This habit carries to Primary 6 and catches a large share of arithmetic slips.

Tutor line: “You say 12 − 7 is 5. Is 7 + 5 equal to 12? Yes, so you are right.”

04 Question types and worked steps

Type	Example	Steps
Take-away: 12 counters, remove 7	subtract down to ten, then the rest, write the answer	written as 12 − 2 − 5
Comparison: how many more is 12 than 7	align two rows, count the excess	same expression, different meaning
Missing number: 12 − ? = 5	ask what plus 5 makes 12	7 + 5 = 12
Word problem: 15 dollars, spend 8	find both numbers, subtract, give the unit	the answer is in dollars

Sanity check (teach this): Keep the minuend at or below 20 and never produce a negative answer; do not use 7 − 12 in this lesson. Contrast take-away and comparison with different story contexts repeatedly, or the pupil will subtract mechanically without a model.

05 Independent practice (3 questions)

Question	Answer	What to watch
12 − 7 = ?	5	12 − 2 − 5
15 − 8 = ?	7	15 − 5 − 3
16 − 9 = ?	7	16 − 6 − 3
Amy has 13, Ben has 8. How many more does Amy have?	5	comparison type
9 + ? = 14	5	think: 14 − 9

06 Common mistakes and what to say

Mistake	Student writes	Say this
Counting back seven places	landing on 6 or 4	“Do not count back. 12 minus 2 reaches 10, then subtract 5.”
Subtracting the larger from the smaller	writing 7 − 12	“Which is bigger? Put the bigger number first.”
Treating a difference as take-away	taking counters away in a difference question	“Nobody removes anything here. It asks how far apart the rows are.”
Leaving out the unit	answering 5 with no unit	“Five what? Say five counters.”

07 Homework (paper, section and question numbers)

Subtraction within 20 practice (in-school) — questions 1–8: take-away and comparison
Primary 1 maths paper 1 — questions 6–10: subtraction word problems

08 Exit check (last 5 minutes)

Question	Answer	Decision
14 − 6 = ?	8	if wrong: reteach subtracting to ten
17 − 9 = ?	8	if wrong: reteach splitting the subtrahend
A has 11, B has 6. How many more?	5	if wrong: reteach comparison type

09 Tutor FAQ

Why not let the pupil count backwards?

Counting back works within 20 but collapses at two-digit subtraction and invites off-by-one slips. Subtracting down to ten extends to numbers within 100.

Should the two meanings be taught together?

No. One expression with two meanings confuses the context. Build the calculation with take-away first, then extend the meaning with comparison.

Must the pupil write the check?

At Primary 1 an oral check is enough. The habit still matters, because it is the most effective way to catch slips.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.