Math Core

Counting back from 15 by 8 is a lot of counting. Here is a better way. Turn the subtraction into an addition question that you already know!

Think addition

What is $12 - 9$ ?

Ask yourself: 9 plus what makes 12?

$9 + 3 = 12$ . So the missing part is 3.

$12 - 9 = 3$ .

Think addition

To find $12 - 9$ , think $9 + \square = 12$ . The number in the box is the answer.

This works because of fact families. 9, 3 and 12 are one family. If $9 + 3 = 12$ , then $12 - 9 = 3$ .

Count up

If you don't know the addition fact, you can count up. For $12 - 9$ , start at 9 and count up to 12: 10, 11, 12. You counted 3 numbers. So $12 - 9 = 3$ .

Worked example: Use a fact you know

What is $15 - 8$ ?

Think: 8 plus what makes 15?

You know $8 + 7 = 15$ . So $15 - 8 = 7$ .

Worked example: Use a double

What is $16 - 8$ ?

Think: 8 plus what makes 16?

$8 + 8 = 16$ is a double. So $16 - 8 = 8$ .

Worked example: Count up

There are 13 kids. 10 are on the swings. How many are not on the swings?

Find $13 - 10$ . Think: 10 plus what makes 13? Count up from 10: 11, 12, 13. That is 3.

$13 - 10 = 3$ . 3 kids are not on the swings.

Common mistake

When you count up, count the steps, not the number you stop on. For $12 - 9$ , you say 10, 11, 12. That is 3 steps, so the answer is 3, not 12.

Tip

Check your answer by adding. $12 - 9 = 3$ is right because $9 + 3 = 12$ .

Practice

Practice 1

What is $11 - 9$ ?

Type a number.

Practice 2

What is $12 - 7$ ?

Type a number.

Practice 3

Which addition fact helps you find $14 - 5$ ?

Practice 4

What is $13 - 7$ ?

Type a number.

Practice 5

What is $14 - 6$ ?

Type a number.

Practice 6

Ava has 15 grapes. She eats 9 of them. How many grapes are left?

Type a number.

Practice 7

What is $18 - 9$ ?

Type a number.