Number Bonds to 20

What are number bonds to 20?

A number bond is a pair of numbers that join together to make a total. In this lesson the total is always 20, so we are hunting for every pair that adds up to 20. These are the number bonds to 20.

The big idea stays the same as smaller bonds:

A part, plus another part, makes the whole.

When the whole is 20, the two parts must fit together perfectly. If you are still building confidence with smaller totals, warm up first with our lesson on number bonds to 10.

The clever shortcut: add 10

Here is the best secret in this whole lesson. You already know the bonds to 10. To turn any bond to 10 into a bond to 20, just add 10 to one of the parts.

• You know 4 + 6 = 10. Add 10 to the 4: 14 + 6 = 20.
• You know 3 + 7 = 10. Add 10 to the 7: 3 + 17 = 20.
• You know 5 + 5 = 10. Add 10 to one 5: 15 + 5 = 20.

So you do not have to learn brand-new facts from scratch. You grow the facts you already know. That is why bonds to 10 are worth learning so well — they unlock the bigger bonds.

All the bonds to 20

Let's list the pairs in order so the pattern jumps out.

One part	Other part	Bond
0	20	0 + 20 = 20
2	18	2 + 18 = 20
4	16	4 + 16 = 20
6	14	6 + 14 = 20
8	12	8 + 12 = 20
10	10	10 + 10 = 20
12	8	12 + 8 = 20
14	6	14 + 6 = 20

Look at the left column — it counts up: 0, 2, 4, 6, 8, 10. Now the right column counts down: 20, 18, 16, 14, 12, 10. As one part grows, the other shrinks by the same amount. That balancing pattern is true for every number bond.

Worked example 1: counting on

You have 15 marbles. How many more do you need to make 20?

Start at 15 and count on to 20, lifting a finger each time: 16, 17, 18, 19, 20. You lifted 5 fingers.

So you need 5 more. 15 + 5 = 20.

Worked example 2: using the add-10 trick

What goes with 7 to make 20?

Think of the bond to 10: 7 needs 3 to make 10. Now make it a bond to 20 by adding 10 to that missing part: 3 + 10 = 13.

Check: 7 + 13 = 20. Correct!

Worked example 3: the fact family

Every bond gives you four facts. From the pair 14 and 6 you get:

• 14 + 6 = 20
• 6 + 14 = 20
• 20 − 6 = 14
• 20 − 14 = 6

Learn one pair and you really know four facts. This group of four is called a fact family.

Why bonds to 20 matter

Twenty is two full tens, so it is a natural stepping stone between 10 and 100. When the bonds to 20 are automatic — your brain just knows that 13 needs 7 — you can add and subtract two-digit numbers in your head much faster. You will use these bonds every time you bridge over a ten or check an answer.

Try it yourself

You will need 20 small objects: pasta pieces, counters or coins.

1. Count out exactly 20 objects and check by counting in twos.
2. Split the pile into two groups any way you like.
3. Count each group and say the bond aloud: "13 and 7 make 20!"
4. Now say the matching subtraction: "20 take away 7 is 13."
5. Challenge: Hide one group under a bowl. Look at the other group and say how many are hidden before you peek.

Do this for a few minutes each day and the bonds will start popping into your head automatically.

What's next?

Once bonds to 20 feel quick, you can use them to jump over tens when you add. Try our lesson on addition and subtraction, or sharpen your speed with mental math strategies.