Math Core

You already know lots of facts. Now you can get fast at all of them, without counting on your fingers. Good math thinkers use a few smart tricks.

Doubles and near doubles

A double adds a number to itself, like $6 + 6 = 12$ .

A near double is one more than a double. For $6 + 7$ , think $6 + 6 = 12$ , then add 1 more. So $6 + 7 = 13$ .

Make a ten

Ten is a friendly number. For $8 + 5$ , 8 needs 2 more to make 10. Break 5 into 2 and 3.

$8 + 2 = 10$ , and $10 + 3 = 13$ .

Think addition to subtract

To find $15 - 7$ , ask: "7 plus what makes 15?"

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

Count back to 10

For $13 - 5$ , first take away 3 to get to 10. Then take away 2 more.

$13 - 3 = 10$ , and $10 - 2 = 8$ . So $13 - 5 = 8$ .

Pick a trick

Two numbers the same or close? Use doubles.
A 9 or 8? Make a ten.
Subtracting? Think addition, or count back to 10.

Worked example: Near double

What is $8 + 9$ ?

$8 + 8 = 16$ . 9 is one more than 8, so add 1.

$8 + 9 = 17$ .

Worked example: Make a ten

What is $9 + 6$ ?

9 needs 1 more to make 10. Break 6 into 1 and 5.

$9 + 1 = 10$ , and $10 + 5 = 15$ . So $9 + 6 = 15$ .

Worked example: Think addition

What is $14 - 6$ ?

Ask: "6 plus what makes 14?" You know $6 + 8 = 14$ .

So $14 - 6 = 8$ .

Common mistake

With a near double, start from the smaller double. For $7 + 8$ , use $7 + 7 = 14$ and add 1 to get 15. If you use $8 + 8 = 16$ , you must take 1 away instead.

Tip

Check a subtraction by adding. If $12 - 5 = 7$ , then $7 + 5$ should be 12. It is!

Practice

Practice 1

What is $7 + 7$ ?

Type a number.

Practice 2

What is $5 + 6$ ?

Type a number.

Practice 3

What is $9 + 7$ ?

Type a number.

Practice 4

Which fact helps you find $17 - 8$ ?

Practice 5

What is $12 - 5$ ?

Type a number.

Practice 6

What is $16 - 7$ ?

Type a number.

Practice 7

Mia has 8 stickers. She gets 5 more. How many stickers does she have now?

Type a number.

Practice 8

There are 15 birds on a fence. 9 fly away. How many birds are still on the fence?

Type a number.