Why Do We Group Numbers?
People have been counting for a very, very long time! Long ago, people used their fingers to count things. We have ten fingers, so people started to group things by tens. That is why our number system is built on the number ten!
So here is the big question: when we count past ten, what happens? How do we make the numbers 11, 12, 13, and all the way to 19? Let's find out!
Core Ideas: Tens and Ones
Every teen number is made of two parts. The first part is always one group of ten. The second part is the extra ones. When you put these two parts together, you get a teen number!
A Group of Ten
Extra Ones
Teen = Ten + Ones
Ones Can Be 1 to 9
See It: Teen Numbers with Blocks
Let's look at a picture! We can use base-ten blocks to see how teen numbers work. A long rod stands for one ten, and small cubes stand for ones.
Do you see the pattern? Every teen number has one tall rod for the ten. Then it has some small cubes for the extra ones. The number of small cubes tells you which teen number it is!
The Math: 10 + ? = Teen Number
We can write a simple math sentence for every teen number. We start with 10, then add the extra ones.
See how easy it is? You just add the ones to 10. The ones digit in a teen number tells you how many extra ones there are. In the number 16, the 6 tells you there are 6 extra ones after the ten!
All Nine Teen Numbers
Let's look at every teen number in a table. Each one follows the same rule: 1 ten + some ones.
| Teen Number | Tens | Ones | Math Sentence |
|---|---|---|---|
| 11 | 1 | 1 | 10 + 1 = 11 |
| 12 | 1 | 2 | 10 + 2 = 12 |
| 13 | 1 | 3 | 10 + 3 = 13 |
| 14 | 1 | 4 | 10 + 4 = 14 |
| 15 | 1 | 5 | 10 + 5 = 15 |
| 16 | 1 | 6 | 10 + 6 = 16 |
| 17 | 1 | 7 | 10 + 7 = 17 |
| 18 | 1 | 8 | 10 + 8 = 18 |
| 19 | 1 | 9 | 10 + 9 = 19 |
Look at the number line above. Every teen number has a label that shows 10 + something. The tens column in our table is always 1. Only the ones column changes!
Worked Example: Breaking Apart 16
Let's work through an example together! We want to break apart the number 16 into tens and ones.
Helpful Tips and Common Mistakes
Let's look at some tips that will help you, and also some mistakes to watch out for.
| Helpful Tip ✅ | Common Mistake ❌ |
|---|---|
| Every teen number always has exactly 1 ten. | Thinking 15 has 15 tens. It only has 1 ten! |
| The ones digit (second number) tells you how many extra ones. | Mixing up tens and ones. In 13, the 3 is the ones, not the 1. |
| Use objects like blocks, crayons, or fingers to check your work. | Forgetting to count the ten. 14 is not just 4 ones — it is 10 + 4. |
| Remember: 10 is NOT a teen number. Teen numbers start at 11. | Calling 10 a teen number. It has 1 ten and 0 ones, but it is not a teen. |
From Teens to Bigger Numbers
Now that you know how teen numbers work, you are ready to learn about even bigger numbers! The same idea of tens and ones keeps going all the way to 99 and beyond.
| What You Know Now | What You'll Learn Next |
|---|---|
| Teen numbers have 1 ten. | Numbers like 20, 30, 40 have 2, 3, or 4 tens! |
| Teen numbers go up to 19. | Two-digit numbers go all the way to 99. |
| 10 + ones = teen number. | Tens + ones works for every two-digit number! 20 + 3 = 23. |
The cool part is that the same rule works! You just change how many tens you have. For teen numbers you always have 1 ten. For numbers in the twenties, you have 2 tens. It's the same idea, just bigger!
Practice Problems
Great job learning about teen numbers! Now let's practice. Try each problem before reading the answer.
Let's Review!
The teen numbers (11 through 19) are all built the same way. Each one has exactly one group of ten plus some extra ones. The number of extra ones can be 1, 2, 3, 4, 5, 6, 7, 8, or 9. We can write a math sentence for any teen number: 10 + ones = teen number.
You can use base-ten blocks, fingers, or drawings to show tens and ones. The ones digit in a teen number tells you how many extra ones there are. This idea of grouping by tens is the start of place value, which you will use for all bigger numbers too!