Adding to 20 without crossing ten

We add bigger numbers, like 12 + 3. The ten has a rest. Only the ones do the counting.

The ten has a rest. Only the leftover counts

You already know how to add to ten. Now we try bigger numbers. Like 12 + 3.

It looks hard. But it is not. Remember: 12 is ten and two. The ten is a finished pile. It does not change. It just lies to the side.

You are really only counting 2 + 3. That is 5. And the ten? You add it back at the end: ten and five is fifteen.

Picture it with bricks. Twelve bricks: ten in a box and two on the table. You add three bricks. Nobody even touched the box — only the table got more. Two and three is five bricks on the table. And the box still holds its ten.

12 + 3 = 15. The whole magic is that you split the big number into a ten and the leftover. The ten sleeps, the leftover counts. Today we will only add questions where the ten does not have to wake up.

Filip and Jonáš collect football cards. Filip has 14 cards. In a new pack he finds 4 more. “I’ll be counting until evening,” he sighs. Jonáš advises: “Put ten cards in the box and shut it. You have four left in your hand. And four new ones. That is eight.” Filip opens the box: “Ten in the box and eight in my hand… eighteen!” He counts them all one by one just to be sure. Eighteen. The box with the ten works. Filip keeps it for the next packs too: ten inside, the rest in his hand. He sees the sum now before Jonáš can open his mouth.

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Split a number past ten: the ten into the box, the leftover in your hand. Add only what you have in your hand.

Adding with a box for the ten

Example: 13 + 5. Step by step:

1. Split the big number. 13 is 10 and 3. 发送 the ten into the box.

2. Add the leftover. In your hand you have 3 and you add 5. You already know that: 3 + 5 = 8.

3. Add the ten back. 10 and 8 is 18.

The writing: 13 + 5 = 18.

Why do we call this “without crossing”? Because the ones together did not grow past nine (3 + 5 = 8, that fits). The ten stayed calm the whole time. If the ones spilled over ten, that would be different work — that comes in the next lesson.

It works the other way too, when the small number is at the front: 4 + 12. Swap: 12 + 4. Split: 10 and 2. Add: 2 + 4 = 6. Add the ten: 16.

And the easiest of all are questions with a full ten: 10 + 7 = 17. You calculate nothing. You just stick the ten and the seven together. Ten and seven is seventeen — the name of the number says it too.

14 + 3 = ?

I split: 14 is 10 and 4. The ten goes into the box.

I add the leftover: 4 + 3 = 7.

I add the ten: 10 and 7 is 17.

14 + 3 = 17.

Check on the line: I stand on 14 and jump three times right: 15, 16, 17. It matches.

10 + 9 = ?

This one is the easiest in the world.

The ten is finished. I add 9 ones.

Ten and nine is nineteen. You hear it in the name: nine-teen, nine and ten.

10 + 9 = 19.

Every ten-plus-something question you solve at once. Just stick the name: ten and three = thirteen, ten and six = sixteen.

2 + 15 = ?

The small number is at the front. No problem — I swap: 15 + 2.

I split: 15 is 10 and 5.

I add the leftover: 5 + 2 = 7.

I add the ten: 17.

2 + 15 = 17. Swapping with plus never spoils anything.

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Watch you do not mix up the writing: the result 17 is written as one and seven, not as 107. The ten and the ones live in one two-digit number.

Practice: now you do it 🎲

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

Add to 20. Put the ten in the box, count the leftover.

Practise the questions

On paper

Paper, a pencil. Draw a box for the ten if you like.

Calculate and write: 12 + 4, 15 + 3, 11 + 7, 13 + 6.
Calculate the flash questions with ten: 10 + 2, 10 + 5, 10 + 8. Just stick the names.
For 3 + 14 swap the numbers first, then calculate.
Check one question on the number line by jumping.

Now you 💪

I can split a number past ten into a ten and the leftover.
Questions like 10 + 6 I solve at once, with no calculating.
I know that I only add the ones and the ten waits to the side.

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Done when: You calculate five questions to 20 without crossing and the writing of the numbers is right.

What to take from this lesson

A number past ten = a ten and the leftover.
The ten has a rest. Only the ones are added.
Ten plus something you can read straight from the name of the number.
Small number at the front? Swap and count from the bigger one.
Without crossing = the ones did not spill past nine.