Subtracting to 20 with crossing

Stop at ten

13 − 5. Five jumps back from thirteen — you can do it, but it is easy to slip. There is a better path. The same as with adding, only backwards.

Ten is a stop. When you jump backwards, stop at ten and take a breath.

13 − 5? From thirteen you first take away 3 — you are on ten. From the five you still have 2 left to take away. And 10 − 2 = 8. Done: 13 − 5 = 8.

Why stop? Because ten is a sure thing. From ten you subtract with no thinking — you know the pairs to ten by heart. Long jumping backwards is like running with your eyes shut. A stop at ten opens your eyes.

Again you split a number into two bits: 5 is 3 and 2. The first bit takes you to ten, the second you finish off. Ten is a solid point. From there, counting is lovely.

Tereza has 14 Kč in her money box. She wants to buy a sticker for 6 Kč. “How much will I have left?” Filip counts backwards on his fingers and gets tangled. Tereza goes through ten: “From fourteen I take four — I am on ten. I still have two to take away. Ten minus two is eight.” Filip does not believe it, so they empty the money box and count six Kč for real. Eight are left. “That stop at ten is really good,” Filip admits. He tries it on the next question at once and for the first time he does not count on his fingers. Tereza lends him a crocodile sticker as a prize.

Subtract with a stop at ten

Example: 15 − 7. Three steps:

1. How many do you take away to get to ten? Fifteen has 5 ones. I take away 5 and I am on 10.

2. Split the number you take away. 7 is 5 and 2. The first 5 I already took away on the way to ten. 2 are left to take away.

3. Take the leftover from ten. 10 − 2 = 8. That is easy — from ten you can subtract using the pairs.

So 15 − 7 = 8.

Writing the method:

15 − 7 = 15 − 5 − 2 = 10 − 2 = 8

There is a second path too, for those who add well: filling up. 15 − 7? Ask: seven and how many is fifteen? Seven and three is ten. Ten and five is fifteen. Together I filled 3 and 5, so 8. Both paths lead to the same result. Pick the one that fits you.

The check is always the same: difference plus the number taken away must give the original. 8 + 7 = 15? Yes. It matches.

12 − 4 = ?

To ten: twelve has 2 ones, I take away 2, I am on 10.

I split the four: 4 is 2 and 2. 2 are left to take away.

From ten: 10 − 2 = 8.

Writing: 12 − 4 = 12 − 2 − 2 = 10 − 2 = 8.

Check: 8 + 4 = 12? Eight, nine, ten, eleven, twelve. Yes.

16 − 9 = ?

To ten: I take away 6, I am on 10.

I split the nine: 9 is 6 and 3. 3 are left to take away.

From ten: 10 − 3 = 7.

16 − 9 = 7.

With nine there is a quick trick too: taking away 9 is almost like taking away 10. Take the whole ten (16 − 10 = 6) and give one back: 6 + 1 = 7. Same result, different path.

11 − 8 = ?

I try filling up. I ask: eight and how many is eleven?

Eight and two is ten. Ten and one is eleven.

I filled 2 and 1, together 3.

11 − 8 = 3.

Check: 3 + 8 = 11? Three plus eight… eight in my head, three jumps: nine, ten, eleven. It matches.

Practice: now you do it 🎲

Now try it for real. You will see all the questions at once, like on paper. Mulai with ten. If it goes well, add more.

Subtract to 20. Stop at ten.

Practise the questions

On paper

Paper, a pencil, and maybe bricks or coins for checking.

1. Calculate with the method: 13 − 6, 15 − 8, 12 − 7. Pattern: 13 − 6 = 13 − 3 − 3 = 10 − 3 = 7.

2. Solve 14 − 9 twice: once with a stop at ten, once with the trick take 10 and give 1 back.

3. Check each result by adding.

4. Make up a word question about crowns in a money box and solve it.

Now you 💪

1. I can stop at ten when I subtract.
2. I can split the number I take away into two bits and not lose any.
3. I can check every difference by adding.

What to take from this lesson

• Subtracting with crossing: stop at ten.
• First take away the ones, then the leftover from ten.
• Split the number you take away and watch both bits.
• Take away 9 = take away 10 and give one back.
• Check by adding: difference + taken away = original number.

© 2026 Ing. Martin Polak / AlgoRhino · Ketentuan penggunaan konten