Place Value Foundations

Unitizing: Making a Ten

Today we will learn about making a ten.

Sometimes we count things one by one. But when we have 10 ones, we can put them together and call them 1 ten. This is called unitizing. That means we take many little things and think of them as one group.

Making a ten helps us count bigger numbers more easily. It also helps us understand two-digit numbers.

Big idea: $$10\text{ ones} = 1\text{ ten}$$

Let’s think about blocks, straws, or dots. If you have 10 single blocks, you can snap them together into 1 long stick. That stick is 1 ten.

So instead of saying “1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ones,” we can say, “I have 1 ten.”

What are ones and tens?

• One means 1 single thing.
• Ten means 1 group of 10 things.

If we have 10 pennies, we can make 1 group of 10 pennies. That is 1 ten pennies, or just 1 ten.

Why making a ten is helpful

• It makes counting faster.
• It helps us see numbers in groups.
• It helps us read and build two-digit numbers.

For example, the number \(14\) means 1 ten and 4 ones.

That is:

$$14 = 1\text{ ten } + 4\text{ ones}$$

How to make a ten

1. Count the ones.
2. When you get to 10 ones, put them in one group.
3. Call that group 1 ten.
4. If there are extra ones, keep them separate.

Let’s practice with some examples.

Example 1: 10 dots

Suppose you see 10 dots.

Count them: \(1, 2, 3, 4, 5, 6, 7, 8, 9, 10\).

Now there are 10 ones. We can make a group.

$$10\text{ ones} = 1\text{ ten}$$

So 10 dots is the same as 1 ten.

Example 2: 12 cubes

Imagine you have 12 single cubes.

First, take 10 cubes and make 1 group of ten.

Then see what is left. There are 2 cubes left.

So \(12\) is:

$$12 = 1\text{ ten } + 2\text{ ones}$$

We say: 12 is 1 ten and 2 ones.

Example 3: 17 straws

You have 17 straws.

Group 10 straws together first. That makes 1 ten.

Now count the straws left over. There are 7 ones left.

So:

$$17 = 1\text{ ten } + 7\text{ ones}$$

We say: 17 is 1 ten and 7 ones.

Example 4: 20 counters

You have 20 counters.

Make one group of 10. That is 1 ten.

There are still 10 more counters. That makes another ten.

So:

$$20 = 2\text{ tens } + 0\text{ ones}$$

We say: 20 is 2 tens and 0 ones.

Let’s look closely at teen numbers

Teen numbers are numbers from 11 to 19.

Each teen number has 1 ten and some ones.

• \(11 = 1\text{ ten } + 1\text{ one}\)
• \(13 = 1\text{ ten } + 3\text{ ones}\)
• \(15 = 1\text{ ten } + 5\text{ ones}\)
• \(19 = 1\text{ ten } + 9\text{ ones}\)

This is why making a ten is so important. It helps us understand all the teen numbers.

Think about groups

If you see many single things, ask yourself:

• Can I make a group of 10?
• How many tens do I have?
• How many ones are left?

These questions help you break a number into tens and ones.

Try this thinking

If you have 16 beads, you can make:

$$16 = 1\text{ ten } + 6\text{ ones}$$

If you have 18 stars, you can make:

$$18 = 1\text{ ten } + 8\text{ ones}$$

If you have 10 apples, you can make:

$$10 = 1\text{ ten } + 0\text{ ones}$$

Important to remember

• 10 ones always make 1 ten.
• A ten is one group, but it has 10 things inside it.
• Numbers bigger than 9 can be shown with tens and ones.

Summary

Making a ten means putting 10 ones together to make 1 ten.

This helps us count, group objects, and understand numbers like 12, 15, and 19.

Remember:

$$10\text{ ones} = 1\text{ ten}$$

When you see a number, try to find the tens and the ones. That is a smart way to understand numbers.

Teen Numbers as One Ten and Ones

Today we will learn about teen numbers. Teen numbers are the numbers from 11 to 19.

These numbers are special because each teen number has 1 group of ten and some extra ones.

Knowing this helps us understand how numbers are built. It also helps us add, count, and work with bigger numbers later.

What is a ten?

A ten is a group of 10 ones. If we put 10 single cubes, sticks, or dots together, we can call that group 1 ten.

So when we look at teen numbers, we can think:

• 1 ten
• and some ones

How teen numbers are made

Every teen number starts with 1 ten. Then we add extra ones.

Here is the pattern:

• 11 is 1 ten and 1 one
• 12 is 1 ten and 2 ones
• 13 is 1 ten and 3 ones
• 14 is 1 ten and 4 ones
• 15 is 1 ten and 5 ones
• 16 is 1 ten and 6 ones
• 17 is 1 ten and 7 ones
• 18 is 1 ten and 8 ones
• 19 is 1 ten and 9 ones

We can also write teen numbers in math form:

$$11 = 10 + 1$$

$$12 = 10 + 2$$

$$13 = 10 + 3$$

$$14 = 10 + 4$$

$$15 = 10 + 5$$

$$16 = 10 + 6$$

$$17 = 10 + 7$$

$$18 = 10 + 8$$

$$19 = 10 + 9$$

A helpful way to think

When you see a teen number, say to yourself:

“1 ten and some ones.”

For example, if you see 16, you can think:

“I know 16 is 1 ten and 6 ones.”

This works for all the teen numbers from 11 to 19.

Worked Example 1

What is 11 made of?

Step 1: Start with 1 ten.

Step 2: Look at the extra ones. For 11, there is 1 one.

So:

$$11 = 10 + 1$$

Answer: 11 is 1 ten and 1 one.

Worked Example 2

What is 14 made of?

Step 1: Teen numbers have 1 ten.

Step 2: 14 has 4 extra ones.

So:

$$14 = 10 + 4$$

Answer: 14 is 1 ten and 4 ones.

Worked Example 3

What number is 1 ten and 7 ones?

Step 1: 1 ten means 10.

Step 2: Add 7 ones.

$$10 + 7 = 17$$

Answer: 1 ten and 7 ones is 17.

Worked Example 4

What number is 1 ten and 9 ones?

Step 1: 1 ten is 10.

Step 2: Add 9 ones.

$$10 + 9 = 19$$

Answer: 1 ten and 9 ones is 19.

How to check your thinking

If you are not sure about a teen number, you can use these steps:

1. Say the number.
2. Remember that all teen numbers have 1 ten.
3. Count the extra ones.
4. Put them together.

Example: For 18:

• 1 ten
• 8 ones

So:

$$18 = 10 + 8$$

Things to remember

• Teen numbers are 11 through 19.
• Each teen number has 1 ten.
• The second part tells how many ones there are.
• Example: 13 means 1 ten and 3 ones.

Summary

Teen numbers are made with one group of ten and some extra ones.

That means:

• 11 is 1 ten and 1 one
• 15 is 1 ten and 5 ones
• 19 is 1 ten and 9 ones

When you see a teen number, think:

“1 ten and some ones.”

Great job learning about teen numbers!

Representing Two-Digit Numbers

Let’s learn how to show two-digit numbers in a simple way.

A two-digit number has two parts: a tens part and a ones part.

We can build two-digit numbers with objects like base-ten blocks or bundled sticks.

• A ten means a group of 10.
• A one means 1 single object.

So, when we see a two-digit number, we can ask:

• How many tens are there?
• How many ones are there?

For example, in the number \(14\):

• The \(1\) means 1 ten.
• The \(4\) means 4 ones.

That means \(14\) is:

$$1\text{ ten }+ 4\text{ ones} = 14$$

Base-ten blocks help us see this clearly.

• A ten rod stands for 10.
• A one cube stands for 1.

Bundled sticks work the same way.

• 1 bundle = 10 sticks
• 1 loose stick = 1

When we make a number, we use the tens first, then the ones.

Main Idea

The digit on the left tells how many tens. The digit on the right tells how many ones.

Let’s look at some numbers:

• \(12\) = 1 ten and 2 ones
• \(25\) = 2 tens and 5 ones
• \(38\) = 3 tens and 8 ones
• \(40\) = 4 tens and 0 ones

If there are no loose ones, we write \(0\) in the ones place.

How to Build a Two-Digit Number

1. Look at the first digit. That tells the number of tens.
2. Look at the second digit. That tells the number of ones.
3. Build the number with tens and ones.

Now let’s try some worked examples.

Example 1: Build \(23\)

The number \(23\) has:

• \(2\) tens
• \(3\) ones

So we build it with:

• 2 ten rods or 2 bundles
• 3 one cubes or 3 loose sticks

We can write:

$$23 = 2\text{ tens }+ 3\text{ ones}$$

Example 2: What number is 1 ten and 6 ones?

1 ten is \(10\).

6 ones is \(6\).

Put them together:

$$10 + 6 = 16$$

So, 1 ten and 6 ones makes \(16\).

Example 3: Build \(34\)

The number \(34\) has:

• \(3\) tens
• \(4\) ones

That means:

$$34 = 3\text{ tens }+ 4\text{ ones}$$

With blocks, we would use:

• 3 ten rods
• 4 one cubes

Example 4: What number is 5 tens and 0 ones?

5 tens means \(50\).

0 ones means no extra ones.

So the number is:

$$50 = 5\text{ tens }+ 0\text{ ones}$$

This is important: a number can have tens even when it has no ones.

Let’s Notice Patterns

• As the tens get bigger, the number gets bigger by 10.
• As the ones get bigger, the number gets bigger by 1.
• Every time we have 10 ones, that is the same as 1 ten.

Here are some more examples:

• \(21\) = 2 tens and 1 one
• \(27\) = 2 tens and 7 ones
• \(30\) = 3 tens and 0 ones
• \(42\) = 4 tens and 2 ones

Try to Think This Way

When you see a two-digit number, say it in parts.

For \(18\), say: 1 ten and 8 ones.

For \(26\), say: 2 tens and 6 ones.

For \(41\), say: 4 tens and 1 one.

This helps you understand what the number really means.

Summary

• A two-digit number has tens and ones.
• The first digit tells the number of tens.
• The second digit tells the number of ones.
• You can show numbers with ten rods and one cubes or bundles and loose sticks.
• Example: \(29\) means 2 tens and 9 ones.

Now you know how to represent two-digit numbers by building them with tens and ones!

Standard Form and Expanded Form

Numbers can be written in different ways. Today we will learn standard form and expanded form.

Standard form is the number written the usual way, like \(34\).

Expanded form shows the parts of the number. It tells how many tens and how many ones are in the number.

For example, in \(34\), the \(3\) means 3 tens and the \(4\) means 4 ones.

That means:

$$34 = 30 + 4$$

So, \(34\) is the standard form, and \(30 + 4\) is the expanded form.

Let’s remember place value:

• The digit on the left in a two-digit number is the tens digit.
• The digit on the right in a two-digit number is the ones digit.

If a number has 2 tens and 5 ones, we can write:

$$25 = 20 + 5$$

This helps us see the number more clearly.

How to change standard form to expanded form

1. Look at the tens digit.
2. Write that many tens.
3. Look at the ones digit.
4. Write that many ones.
5. Put a plus sign in the middle.

Example: \(47\)

• \(4\) tens = \(40\)
• \(7\) ones = \(7\)

So:

$$47 = 40 + 7$$

How to change expanded form to standard form

1. Look at the tens number.
2. Look at the ones number.
3. Put them together to make one two-digit number.

Example: \(50 + 2\)

• \(50\) means 5 tens
• \(2\) means 2 ones

So:

$$50 + 2 = 52$$

Worked Examples

Example 1: Write \(18\) in expanded form.

The number \(18\) has:

• \(1\) ten = \(10\)
• \(8\) ones = \(8\)

So:

$$18 = 10 + 8$$

Example 2: Write \(63\) in expanded form.

The number \(63\) has:

• \(6\) tens = \(60\)
• \(3\) ones = \(3\)

So:

$$63 = 60 + 3$$

Example 3: Write \(20 + 6\) in standard form.

\(20\) means 2 tens. \(6\) means 6 ones.

Put them together:

$$20 + 6 = 26$$

Example 4: Write \(70 + 9\) in standard form.

\(70\) means 7 tens. \(9\) means 9 ones.

Put them together:

$$70 + 9 = 79$$

Tips to help you

• Think: tens first, ones next.
• The tens digit tells how many groups of ten.
• The ones digit tells how many extra ones.
• Expanded form always shows the tens part and the ones part.

Let’s look at one more set:

• \(42\) in expanded form is $$40 + 2$$
• \(30 + 5\) in standard form is $$35$$

Brief Summary

Standard form is the number written normally, like \(56\).

Expanded form shows the tens and ones, like:

$$56 = 50 + 6$$

When you see a two-digit number, think about its tens and ones. That will help you write the number in both forms.

Comparing Two-Digit Numbers

Today we will learn how to compare two-digit numbers.

A two-digit number has a tens place and a ones place. We use the tens first. If the tens are the same, then we look at the ones.

Comparing means telling which number is greater, which number is less, or if the numbers are equal.

We can use these math signs:

• > means greater than
• < means less than
• = means equal to

For example:

• \(45 > 32\) means 45 is greater than 32.
• \(18 < 21\) means 18 is less than 21.
• \(56 = 56\) means 56 is equal to 56.

Step 1: Look at the tens place.

The tens place tells how many groups of 10 are in the number.

In the number \(34\), the 3 means 3 tens and the 4 means 4 ones.

In the number \(58\), the 5 means 5 tens and the 8 means 8 ones.

If one number has more tens, it is the greater number.

For example, compare \(42\) and \(67\).

• \(42\) has 4 tens.
• \(67\) has 6 tens.

Since 6 tens is more than 4 tens, we know:

$$67 > 42$$

Step 2: If the tens are the same, look at the ones place.

The ones place tells how many extra ones are in the number.

For example, compare \(53\) and \(58\).

• Both numbers have 5 tens.
• Now look at the ones: 3 ones and 8 ones.

Since 8 ones is more than 3 ones, we know:

$$58 > 53$$

Step 3: If the tens and ones are both the same, the numbers are equal.

For example, compare \(26\) and \(26\).

• Both have 2 tens.
• Both have 6 ones.

So the numbers are the same:

$$26 = 26$$

Think about tens and ones

• A number with more tens is greater.
• If the tens match, a number with more ones is greater.
• If both match, the numbers are equal.

Worked Example 1

Compare \(24\) and \(31\).

First look at the tens.

• \(24\) has 2 tens.
• \(31\) has 3 tens.

3 tens is more than 2 tens, so:

$$24 < 31$$

Worked Example 2

Compare \(46\) and \(43\).

First look at the tens.

• \(46\) has 4 tens.
• \(43\) has 4 tens.

The tens are the same, so look at the ones.

• \(46\) has 6 ones.
• \(43\) has 3 ones.

6 ones is more than 3 ones, so:

$$46 > 43$$

Worked Example 3

Compare \(70\) and \(68\).

First look at the tens.

• \(70\) has 7 tens.
• \(68\) has 6 tens.

7 tens is more than 6 tens, so:

$$70 > 68$$

Worked Example 4

Compare \(55\) and \(55\).

First look at the tens.

• Both numbers have 5 tens.

Now look at the ones.

• Both numbers have 5 ones.

The numbers are equal:

$$55 = 55$$

A good way to remember

1. Look at the tens.
2. If the tens are the same, look at the ones.
3. Choose >, <, or =.

Try saying it like this:

• "\(39\) is less than \(41\)."
• "\(62\) is greater than \(29\)."
• "\(44\) is equal to \(44\)."

Summary

To compare two-digit numbers, look at the tens place first. The number with more tens is greater. If the tens are the same, look at the ones place. The number with more ones is greater. If both places are the same, the numbers are equal.

You can do it!

Using Comparison Symbols: <, >, =

Today we will learn how to compare two numbers using special math symbols.

These symbols are:

• Greater than: \(>\)
• Less than: \(<\)
• Equal to: \(=\)

We use these symbols to tell if one number is bigger, smaller, or the same as another number.

What the symbols mean

• \(>\) means greater than. The number on the left is more.
• \(<\) means less than. The number on the left is smaller.
• \(=\) means equal to. Both numbers are the same.

You can think of \(<\) and \(>\) like a hungry alligator. The alligator wants to eat the bigger number, so its open mouth points to the bigger number.

For example:

$$5 > 3$$

This means 5 is greater than 3.

And:

$$2 < 6$$

This means 2 is less than 6.

And:

$$4 = 4$$

This means 4 is equal to 4.

How to compare numbers

When we compare numbers, we ask:

• Which number is bigger?
• Which number is smaller?
• Are they the same?

If you are comparing small numbers, you can count objects or think about which number comes later when counting.

If you are comparing two-digit numbers, it helps to think about tens and ones.

A two-digit number has:

• Tens  groups of 10
• Ones  extra single ones

For example, \(14\) has 1 ten and 4 ones. \(23\) has 2 tens and 3 ones.

When comparing two-digit numbers:

1. First compare the tens.
2. If the tens are the same, compare the ones.

Worked Example 1

Compare \(3\) and \(7\).

We know 3 is smaller than 7.

So we write:

$$3 < 7$$

Worked Example 2

Compare \(6\) and \(6\).

Both numbers are the same.

So we write:

$$6 = 6$$

Worked Example 3

Compare \(12\) and \(18\).

Look at the tens first.

Both numbers have 1 ten.

Now look at the ones.

\(12\) has 2 ones. \(18\) has 8 ones.

Since 2 ones is less than 8 ones, 12 is less than 18.

So we write:

$$12 < 18$$

Worked Example 4

Compare \(27\) and \(19\).

Look at the tens first.

\(27\) has 2 tens. \(19\) has 1 ten.

Since 2 tens is more than 1 ten, 27 is greater than 19.

So we write:

$$27 > 19$$

Tips to help you remember

• The open side of \(<\) or \(>\) faces the bigger number.
• The pointy side faces the smaller number.
• If the numbers are the same, use \(=\).
• For two-digit numbers, compare tens first, then ones.

Lets look at a few more comparisons

$$9 > 4$$

9 is greater than 4.

$$11 = 11$$

11 is equal to 11.

$$15 < 20$$

15 is less than 20.

In \(15\), there is 1 ten and 5 ones. In \(20\), there are 2 tens and 0 ones. Since 1 ten is less than 2 tens, 15 is less than 20.

Summary

We use comparison symbols to show how two numbers are related.

• Use \(>\) when the first number is greater than the second number.
• Use \(<\) when the first number is less than the second number.
• Use \(=\) when both numbers are equal.

When comparing two-digit numbers, remember:

1. Compare the tens.
2. If the tens are the same, compare the ones.

With practice, you will get very good at using \(<\), \(>\), and \(=\)!

Mental Math: Ten More and Ten Less

Today we will learn a quick way to work with two-digit numbers. We will learn how to find ten more and ten less in our heads.

This is called mental math. Mental math means you solve the problem in your mind without counting one by one.

To understand ten more and ten less, we need to think about tens and ones.

A two-digit number has:

• a tens digit
• a ones digit

For example, in the number \(34\):

• The \(3\) means 3 tens.
• The \(4\) means 4 ones.

So \(34\) is:

When we add or subtract 10, something special happens:

• The tens digit changes.
• The ones digit stays the same.

That is because \(10\) is 1 ten.

If we add 10, we add 1 more ten.

If we subtract 10, we take away 1 ten.

Important idea:

• Ten more means the tens digit goes up by 1.
• Ten less means the tens digit goes down by 1.
• The ones digit does not change.

Let’s look at some numbers in a simple way:

• \(25\) has 2 tens and 5 ones.
• Ten more than \(25\) is \(35\).
• Ten less than \(25\) is \(15\).

See what happened?

• The tens digit changed: \(2 \to 3\) or \(2 \to 1\).
• The ones digit stayed \(5\).

How to find ten more or ten less

1. Look at the two-digit number.
2. Find the tens digit.
3. Keep the ones digit the same.
4. For ten more, add 1 to the tens digit.
5. For ten less, subtract 1 from the tens digit.

Let’s practice with worked examples.

Example 1: Find ten more than \(12\)

The number \(12\) has 1 ten and 2 ones.

Adding 10 means adding 1 more ten.

So the tens digit goes from \(1\) to \(2\). The ones digit stays \(2\).

So, ten more than \(12\) is \(22\).

Example 2: Find ten less than \(47\)

The number \(47\) has 4 tens and 7 ones.

Taking away 10 means taking away 1 ten.

So the tens digit goes from \(4\) to \(3\). The ones digit stays \(7\).

So, ten less than \(47\) is \(37\).

Example 3: Find ten more and ten less than \(56\)

The number \(56\) has 5 tens and 6 ones.

For ten more, the tens digit goes up by 1:

For ten less, the tens digit goes down by 1:

So:

• Ten more than \(56\) is \(66\)
• Ten less than \(56\) is \(46\)

Example 4: Find ten more and ten less than \(80\)

The number \(80\) has 8 tens and 0 ones.

The ones digit is \(0\), and it stays the same.

Ten more:

Ten less:

So:

• Ten more than \(80\) is \(90\)
• Ten less than \(80\) is \(70\)

Let’s notice a pattern

• \(13 \to 23 \to 33\)
• \(41 \to 51 \to 61\)
• \(28 \to 38 \to 48\)

In each pattern, the ones digit stays the same. Only the tens digit changes.

Helpful tip

If you know the number, you do not need to count 10 steps. Just change the tens digit by 1.

For example:

• Ten more than \(62\) is \(72\).
• Ten less than \(62\) is \(52\).

Watch out

• Do not change the ones digit.
• For \(39\), ten more is \(49\), not \(40\).
• For \(64\), ten less is \(54\), not \(63\).

Try thinking like this:

• \(27\): 2 tens, 7 ones
• Ten more: 3 tens, 7 ones = \(37\)
• Ten less: 1 ten, 7 ones = \(17\)

Summary

When you find ten more or ten less, think about tens and ones.

• A two-digit number has tens and ones.
• Adding 10 means adding 1 more ten.
• Subtracting 10 means taking away 1 ten.
• The tens digit changes.
• The ones digit stays the same.

Now you can use mental math to solve problems quickly:

• \(31 + 10 = 41\)
• \(31 - 10 = 21\)
• \(74 + 10 = 84\)
• \(74 - 10 = 64\)

Great job! You are learning how numbers work with tens and ones, and that helps you do math faster.

Mental Math: One More and One Less

Today we will learn how to find one more and one less than a number.

This means we look for the number right after it or right before it. We can do this in our heads with mental math.

When we count forward, we get one more. When we count backward, we get one less.

For example, if we have 14:

• One more than 14 is 15.
• One less than 14 is 13.

These numbers are neighbors on the number line. The number in the middle is the number we start with.

Let’s think about tens and ones.

Two-digit numbers have tens and ones.

• In 23, the 2 means 2 tens.
• In 23, the 3 means 3 ones.

So 23 is 2 tens and 3 ones.

Most of the time, when we find one more or one less, only the ones change.

For example:

• 24 is 2 tens and 4 ones.
• One more is 25, which is 2 tens and 5 ones.
• One less is 23, which is 2 tens and 3 ones.

The tens stayed the same. Only the ones changed by 1.

How to find one more

1. Say the number.
2. Count forward 1.
3. That new number is one more.

Example: Start at 31. Count forward one step: 32. So, one more than 31 is 32.

How to find one less

1. Say the number.
2. Count backward 1.
3. That new number is one less.

Example: Start at 31. Count backward one step: 30. So, one less than 31 is 30.

Watch out at the end of a ten.

Sometimes the ones digit is 9 or 0. Then the tens and ones can both change.

For example, think about 29.

• 29 is 2 tens and 9 ones.
• One more than 29 is 30.

Now the number becomes 3 tens and 0 ones.

Think about 40.

• 40 is 4 tens and 0 ones.
• One less than 40 is 39.

Now the number becomes 3 tens and 9 ones.

This is why knowing tens and ones helps us.

Worked Examples

Example 1: Find one more and one less than 18.

18 is 1 ten and 8 ones.

• One more: count forward from 18 to 19.
• One less: count backward from 18 to 17.

So:

$$18 + 1 = 19$$

$$18 - 1 = 17$$

Example 2: Find one more and one less than 42.

42 is 4 tens and 2 ones.

• One more is 43.
• One less is 41.

The tens stay the same. The ones change.

$$42 + 1 = 43$$

$$42 - 1 = 41$$

Example 3: Find one more than 29 and one less than 29.

29 is 2 tens and 9 ones.

• One more than 29 is 30.
• One less than 29 is 28.

For one more, we move to the next ten.

$$29 + 1 = 30$$

$$29 - 1 = 28$$

Example 4: Find one more and one less than 50.

50 is 5 tens and 0 ones.

• One more is 51.
• One less is 49.

For one less, we move back to the ten before.

$$50 + 1 = 51$$

$$50 - 1 = 49$$

Tips to help you

• One more means the next number.
• One less means the number before.
• Count forward for one more.
• Count backward for one less.
• Look at the ones digit first.

If the ones digit is not 9 or 0, the tens usually stay the same.

If the ones digit is 9 and you add 1, you move to the next ten.

If the ones digit is 0 and you subtract 1, you move to the ten before.

Try thinking about these:

• One more than 16 is 17.
• One less than 16 is 15.
• One more than 67 is 68.
• One less than 67 is 66.
• One more than 39 is 40.
• One less than 70 is 69.

Summary

To find one more, count forward 1. To find one less, count backward 1.

Two-digit numbers are made of tens and ones. Most of the time, only the ones change. But when a number ends in 9 or 0, the tens can change too.

With practice, you can find one more and one less quickly in your head.