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Basic Arithmetic

# Basic Arithmetic

### Basic Arithmetic

Addition, subtraction, multiplication and division are called operations. We can use them to manipulate numbers.

1

These symbols will be familiar to you. How else can we describe them? They are _________

2

The four operations are:

Addition $+$ Subtraction $-$ Multiplication $\times$ Division $\div$

3

Starting with addition, what is $17+8$?

1

What if you have $170$ and you add $27$?

Let's work it out step by step ➡️

2

First break $27$ into easier numbers

$27$ is made up of $2$ tens and $7$ units

3

Add $20$ onto $170$

4

Now add $7$ to $190$

5

So in total...

$170+27=197$

1

The order of the numbers doesn't matter.

2

What is $14+7$?

3

We've seen that $14+7=21$. Let's change the order of our sum, what is $7+14$?

4

We have shown that $14+7=7+14$.

This is true for all additions.

What is $87+115$? Remember, if it makes it easier you can work out $115+87$ as addition is commutative.

1

Sometimes it's easier to add more than we need

then subtract the difference.

2

What about $143+48$?

Let's work it out step by step ➡️

3

$48$ is two less than $50$

So $48$ becomes $50-2$

4

Add $50$ to $143$

5

Now subtract $2$ from $193$

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The final answer is $191$

Nice! 👍

What is $122+79$?

1

What is the opposite of addition?

2

Subtraction has similar strategies to addition

We can also use the words take away or minus for subtraction.

3

Take care with subtraction, unlike addition it is not commutative.

The order of the numbers matters $14-6=8$ but $6-14=-8$.

4

What about $186 - 72$?

Let's work it out step by step ➡️

5

Let's break $72$ into smaller parts

$72$ = $7$ tens and $2$ units

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Subtract $7$ tens from $186$

7

Now subtract $2$ units from $116$

8

The final answer is $114$

Great work! 👍

Calculate $187 - 53$

What is $203-79$?

Multiplication uses our knowledge of times tables. What is $8\times 4$?

Let's try a harder multiplication: $17 \times 8$

1

We can split this into two multiplications

Let's work it out step by step ➡️

2

Break $17$ into smaller parts $17=10+7$.

We can then work out $10\times 8$ and add it to $7\times 8$.

3

First, work out $10 \times 8$

4

Now work out $7 \times 8$

5

$80 + 56 = 136$

6

The final answer is $136$

Nice!

Like addition, multiplication is commutative. What does this mean?

What is $7\times 23$? Remember that you can change the order of the numbers if it makes it easier.

What is the opposite of multiplication?

For example: $18 \div 3$

1

$18$ can be split into groups of $3's$

We use our knowledge of the times tables, in this case$6 \times 3 = 18$

2

So six groups of three go into $18$ or:

$18 \div 3 = 6$

Work out $45 \div 9$

Which multiplication fact can be used to work out $72 \div 12$?

Like subtraction, division is not commutative. The order of the numbers matter. $8\div 4=2$ but $4\div 8=0.5$.

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Summary! The four operations are:

Addition $+$ Subtraction $-$ Multiplication $\times$ Division $\div$

2

Splitting a number $170+27=170+20+7$ Adding on more $143+48=143+50-2$

3

Strategies for subtraction include:

Splitting a number $187-53=187-50-3$ Taking away more $357-49=357-50+1$

4

Strategies for multiplication include:

Splitting a number $34\times 9=30\times 9+4\times 9$

5

Strategies for division include:

Use knowledge of times tables $84\div 7=12$ as $7\times 12=84$.

6

The order of the numbers doesn't matter $115+80=80+115$
$15\times 3=3\times 15$
The order of the numbers does matter $10-2=8$ but $2-10=-8$
$40\div 10=4$ but $10\div 40=\dfrac{1}{4}$