Understanding Teens - 1st Grade Math

SECTION 1

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!

Long Ago • Counting on Fingers

People used their ten fingers to count sheep, rocks, and food.

Later • Tally Marks

People scratched lines on bones and sticks to keep track of bigger numbers.

Even Later • Grouping by Tens

People found it easier to make groups of ten. One group of ten is faster to count than ten single marks!

Today • Our Number System

We use tens and ones every day. The teen numbers (11–19) all use one group of 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!

SECTION 2

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

When you have 10 things, you can bundle them into one group of ten. Think of a full egg carton row or ten crayons in a box.

Extra Ones

After you make your group of ten, you might have some leftover ones. These are the ones that don't fit in the group of ten.

Teen = Ten + Ones

A teen number always equals 1 ten plus some ones. For example, 14 = 1 ten + 4 ones.

Ones Can Be 1 to 9

The extra ones part can be 1, 2, 3, 4, 5, 6, 7, 8, or 9. That gives us the numbers 11 through 19!

✦

KEY TAKEAWAY

Think of a teen number like a lunch box. The big sandwich is the group of ten — it is always there. The little snacks are the extra ones. If you have 1 sandwich and 3 snacks, that is like 10 + 3 = 13!

SECTION 3

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.

Each tall blue rod is one group of ten. The small pink cubes are the extra ones. Notice every teen number has exactly one rod!

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!

SECTION 4

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.

TEEN NUMBER RULE

10 + ones = teen number

The word ones can be 1, 2, 3, 4, 5, 6, 7, 8, or 9.

EXAMPLE: 14

10 + 4 = 14

14 is made of 1 ten and 4 ones.

EXAMPLE: 17

10 + 7 = 17

17 is made of 1 ten and 7 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!

SECTION 5

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

All teen numbers from 11 to 19

This number line shows all the teen numbers from 11 to 19. Above each number, you can see the addition sentence: 10 + ones.

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!

SECTION 6

Worked Example: Breaking Apart 16

Let's work through an example together! We want to break apart the number 16 into tens and ones.

Break Apart 16

1
Step 1 — Is 16 a teen number?

Yes! 16 is between 11 and 19, so it is a teen number. That means it has 1 ten and some ones.
2
Step 2 — Find the ten

Every teen number has exactly 1 ten. That is the 1 at the front of 16. So we have 10.

Tens: 1 ten = 10
3
Step 3 — Find the ones

Look at the second digit in 16. It is 6. That tells us there are 6 ones.

Ones: 6
4
Step 4 — Write the math sentence

Put the ten and the ones together: 10 + 6 = 16. We did it!

16 = 10 + 6 → 1 ten and 6 ones

SECTION 7

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.

Tips vs. Common Mistakes

✦

KEY TAKEAWAY

Think of teen numbers like a bus! The bus is the group of ten — it always carries 10 kids. The extra kids waiting at the bus stop are the ones. If 6 kids are waiting, that is 10 + 6 = 16 kids in all!

SECTION 8

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.

Teen Numbers vs. Bigger Numbers

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!

SECTION 9

Practice Problems

Great job learning about teen numbers! Now let's practice. Try each problem before reading the answer.

Question 1: CONCEPTUAL

Is the number 14 made of tens and ones? If yes, how many tens and how many ones does it have?

Show answer

Yes! The number 14 is a teen number, so it is made of tens and ones. It has 1 ten and 4 ones. You can think of it as a group of 10 blocks plus 4 extra blocks. We write the math sentence as 10 + 4 = 14.

Question 2: BASIC CALCULATION

Fill in the blank: 10 + ___ = 12. How many ones does 12 have?

Show answer

The answer is 2. The number 12 is made of 1 ten and 2 ones. So 10 + 2 = 12. The ones digit in 12 is 2, and that tells you the answer!

Question 3: INTERMEDIATE

Sam has 1 group of ten stickers and 8 extra stickers. How many stickers does Sam have in all? Write the math sentence.

Show answer

Sam has 18 stickers. The math sentence is 10 + 8 = 18. Sam's 1 group of ten equals 10, and the 8 extra stickers are the ones. Put them together and you get the teen number 18.

Question 4: APPLIED

Maya has 15 crayons. She puts 10 in a box. How many crayons are left outside the box? Show how 15 breaks into tens and ones.

Show answer

There are 5 crayons left outside the box. The number 15 is made of 1 ten and 5 ones. When Maya puts the 10 crayons (the ten) in the box, the 5 extra ones are left outside. We write 15 = 10 + 5.

Question 5: CRITICAL THINKING

Jaden says that the number 20 is a teen number because it has the word 'twenty' which sounds like 'teen.' Is Jaden right? Why or why not?

Show answer

Jaden is not right. Teen numbers go from 11 to 19. They all have exactly 1 ten and some ones (1 to 9). The number 20 has 2 tens and 0 ones, so it is not a teen number. Even though 'twenty' might sound a bit like 'teen,' it has a different number of tens!

SUMMARY

Let's Review!

Summary

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!