Quick subtraction within 20 (Primary 1 interviews): Master carrying with finger addition and tackle preschool maths interview questions! 🧮🎓

Published : 07/10/2026

When preparing for Primary 1 interviews, many parents focus on teaching their children to add and subtract numbers within 20, alongside Chinese, English and general knowledge.

With questions like “12−8” and “17−9”, some parents want their child to memorise the answers.

But the question is:

👉 If the interviewer rephrases the question slightly, can the child still work it out?

The ideal maths practice isn’t about making children memorise answers; it’s about helping them understand the relationships between numbers.

Here’s a simple way to teach children subtraction within 20, then gradually extend it to two-digit subtraction.

文章目錄

✋ Step 1: Use both hands to represent 0–10

We can start by turning fingers into a “number tool” that’s easy for children to understand.

One hand can represent 0–10

- 👊 Fist = 10

- ☝️ Index finger = 1

- ✌️ Index finger + middle finger = 2

- 🤟 Index + middle + ring fingers = 3

- 🖐️ Four fingers = 4

- 👍 Thumb = 5

- 👍☝️ Thumb + index finger = 6

- 👍✌️ Thumb + index finger + middle finger = 7

- 👍🤟 Thumb + index finger + middle finger + ring finger = 8

- 🖐️ Five fingers = 9

🧠 Step 2: How can you work out 12−8 quickly? (Two-digit minus one-digit｜Regrouping subtraction)

For example:

12−8＝?

We can think of 12 as:

10＋2

In other words:

👊 ＋ ✌️

And 8 is:

🖐️＋🤟

This is where a simple concept called Simplifying can be used.

First, subtract 2 from both sides:

12－8

↓

（12－2）－（8－2）

↓

10−6

↓

✅ Answer＝4

Your child doesn’t need to memorise “12−8＝4”; they just need to understand:

«“Subtracting the same number from both sides leaves the answer unchanged.”»

This is the mathematical concept of simplification.

2. 🔢 Step 3: 22−18 can actually become 12−8 (Two-digit minus two-digit｜Regrouping subtraction)

When children start learning two-digit subtraction, many parents find that:

22−18

It seems much harder than subtraction within 20.

You can break down the numbers:

22＝10＋10＋2

18＝10＋8

Both numbers have a “10” in them.

So first, you can cancel out the 10 that both numbers have in common:

22−18

↓

12－8

And we’ve already learned:

12−8＝4

So:

🎯 22－18＝4

This way, your child won’t need to memorise another two-digit subtraction problem.

Instead, turn a harder problem into one you already know how to solve.

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3. ⭐ Try Another One: 22－12 (Two Two-Digit Numbers｜Subtraction Without Regrouping)

Now take another look:

22－12＝？

22 can be broken down into:

10＋10＋2

12 can be broken down into:

10＋2

Likewise, cancel out the 10 on both sides:

22−12

↓

12－2

↓

✅ 10

Seems simple, right?

This is the kind of mathematical thinking we want children to develop:

It’s not about “memorising the answer”; it’s about “knowing how to make a problem simpler.”

4. 🎓 Primary One Interviews: The Biggest Mistake Is When Children Only Know How to “Memorise Answers”

When preparing for Primary 1 interviews, many parents want their child to quickly memorise:

1+1, 2+2, 5+3, 8+8……

This kind of practice can help children calculate faster.

But proper preparation for Primary 1 interviews goes beyond simply “memorising answers.”

Because what if the interviewer asks:

8 + 8 = ?

The child answers:

16!

No problem.

But what if the teacher suddenly asks it another way:

❓ + 2 = 9

Would your child know the answer?

The answer is:

At this point, the teacher isn’t just checking whether children have memorised “7+2=9”.

Instead:

« Do children really understand the relationships between numbers?»

Olympiad Maths Games Class（ English Olympiad Maths ）

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Combine different types of mathematical concepts
Applying maths to everyday life

5. 🚨 Are Primary 1 interviews really just about subtraction within 20?

Not necessarily.

When preparing for Primary 1 interviews, many parents focus on practising:

“What is 1＋2?”

「8－3＝？」

「12－5＝？」

Or have your child memorise loads of two-digit addition and subtraction problems.

But what if the interviewer changes the question slightly?

❓“＋2＝9, so what is ？”

Or:

🔢 “Counting back from 120”

Or ask it as a game:

🐸 Little Frog jumps forward＝3

🐸 Hop＝2

🐸 Step＝1

So:

Jump＋Hop＋Step

Where will the little frog end up?

At this point, children who have only memorised subtraction facts within 20 may not be able to answer straight away.

What the interviewer may really want to know is not:

«❌ “How many sums have you memorised?”»

Instead:

«✅ “Do you really understand numbers?”»

6. 🎯 Primary 1 Interview Prep: Memorising Answers vs. Understanding Maths

Parents might want to consider:

If children only remember:

12−8＝4

When the question becomes:

22－18＝？

You may have to learn it all over again.

But if your child understands:

22－18 → 12－8 → 4

Then they’ll know how to work out new problems on their own.

This skill can also be applied to:

🔹 Addition Within 20

🔹 Subtraction within 20

🔹 Two-Digit Addition and Subtraction

🔹 Counting back

🔹 Number Relationships

🔹 Comparing Quantities

🔹 Mental arithmetic

🔹 Number Patterns

🔹 Maths Puzzles for Interviews

So, Primary 1 interview prep should go beyond practising sums.

7. What is mathematics? What is arithmetic?

Maths is more than addition, subtraction, multiplication and division. It’s a science—and clearly, neither abacus mental arithmetic nor Kumon is a science! That’s why these are called arithmetic, not maths.

If you want to know what mathematics really is, you can read a book called A Mathematician’s Lament.

P.S. Using Montessori math materials is real mathematics. Math Olympiad is real mathematics too.

Reference: Curious Mum

8. 👀 If you just want to learn subtraction within 20

If your goal is simple:

👉 Help your child master subtraction within 20 in a short time

You can start by using fingers to represent numbers, breaking numbers apart and Simplifying, so children can learn basic calculation strategies.

But if you want your child to feel confident in Primary 1 interviews:

✅ More than just memorising answers

✅ Work out the answer even when the question changes

✅ Understand number relationships

✅ Answer follow-up questions from the interviewer without getting caught out

✅ Apply maths concepts to different types of questions

The next step, then, goes beyond subtraction within 20.

👉 Instead, build a complete mathematical way of thinking.