Math Core

How much is $4 \times 30$ ? It looks big, but you can solve it with a fact you already know. The trick is to think in tens.

Count tens, like counting anything

Numbers like 10, 20, 30, …, 90 are called multiples of 10. Each one is a number of tens:

30 is 3 tens.
70 is 7 tens.

So $4 \times 30$ means 4 groups of 3 tens.

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Each 🟩 is a ten. There are $4 \times 3 = 12$ tens. And 12 tens is 120.

4 \times 30 = 4 \times 3 \text{ tens} = 12 \text{ tens} = 120

It works like counting apples. 4 groups of 3 apples is 12 apples. 4 groups of 3 tens is 12 tens.

Use the associative property

You can show the same idea with the associative property. Write 30 as $3 \times 10$ :

4 \times 30 = 4 \times (3 \times 10) = (4 \times 3) \times 10 = 12 \times 10 = 120

First do the small fact, $4 \times 3 = 12$ . Then multiply by 10 by putting a 0 on the end.

Multiplying by a multiple of 10

To find $4 \times 30$ :

Use the fact without the zero: $4 \times 3 = 12$ .
That tells you how many tens: 12 tens.
Write 12 tens as a number: 120.

Look at the pattern:

Fact	Tens	Answer
$6 \times 2 = 12$	$6 \times 2$ tens	$6 \times 20 = 120$
$6 \times 5 = 30$	$6 \times 5$ tens	$6 \times 50 = 300$
$6 \times 8 = 48$	$6 \times 8$ tens	$6 \times 80 = 480$

Worked example: Tens with a small fact

What is $2 \times 40$ ?

$2 \times 4 = 8$ , so $2 \times 40$ is 8 tens.

$2 \times 40 = 80$ .

Worked example: Tens with a bigger fact

What is $7 \times 60$ ?

$7 \times 6 = 42$ , so $7 \times 60$ is 42 tens.

$7 \times 60 = 420$ .

Worked example: When the fact already ends in 0

What is $5 \times 80$ ?

$5 \times 8 = 40$ , so $5 \times 80$ is 40 tens. 40 tens is 400.

$5 \times 80 = 400$ .

Common mistake

Watch out when the fact already ends in 0. $5 \times 80$ is 400, with two zeros. One zero comes from the fact ( $5 \times 8 = 40$ ). The other zero comes from the tens. Don't write 40!

Tip

Check your size. $3 \times 90$ should be close to $3 \times 100 = 300$ . If you get 27 or 2,700, it's way off. The answer is 270.

Practice

Practice 1

What is $3 \times 20$ ?

Type a number.

Practice 2

What is $4 \times 40$ ?

Type a number.

Practice 3

What is $9 \times 70$ ?

Type a number.

Practice 4

Which is equal to $8 \times 30$ ?

Practice 5

What is $6 \times 50$ ?

Type a number.

Practice 6

Fill in the missing number: $\underline{\quad} \times 60 = 480$ .

Type a number.

Practice 7

A box of pencils has 40 pencils. How many pencils are in 7 boxes?

Type a number.