Math Core

Sometimes a number in an equation is hiding. A box $\square$ shows where it goes. Your job is to find the number that makes the equation true.

What goes in the box?

Look at $5 + \square = 8$ .

You need a number that makes both sides worth 8. Start at 5 and count on to 8: six, seven, eight. That is 3 more.

So $5 + 3 = 8$ . The missing number is 3.

Make both sides the same

The missing number is the one that makes the equation true. Both sides must be worth the same amount.

Tools you already know

Count on from the smaller number to the bigger one.
Think addition to subtract: for $\square - 4 = 5$ , ask "what is $5 + 4$ ?"
Use a fact family: $6 + 7 = 13$ , so $13 - 7 = 6$ .

Worked example: Missing part of a sum

Find the missing number: $\square + 4 = 11$ .

Ask: what plus 4 makes 11? Count back 4 from 11: ten, nine, eight, seven.

The missing number is 7. Check: $7 + 4 = 11$ . ✅

Worked example: Missing number in subtraction

Find the missing number: $12 - \square = 9$ .

Ask: $9 + \square = 12$ ? Count on from 9: ten, eleven, twelve. That is 3.

The missing number is 3. Check: $12 - 3 = 9$ . ✅

Worked example: Math on both sides

Find the missing number: $6 + 2 = \square + 5$ .

First find the left side: $6 + 2 = 8$ .

Now the right side must be 8 too. What plus 5 makes 8? That is 3.

Check: $6 + 2 = 8$ and $3 + 5 = 8$ . ✅

Common mistake

In $6 + 2 = \square + 5$ , the box is not 8! Many people put the answer to $6 + 2$ in the box. But then the right side would be $8 + 5 = 13$ . Make both sides match.

Tip

Always check. Put your number in the box and see if both sides match.

Practice

Practice 1

Find the missing number: $3 + \square = 7$

Type a number.

Practice 2

Find the missing number: $\square + 4 = 10$

Type a number.

Practice 3

Find the missing number: $9 - \square = 4$

Type a number.

Practice 4

Find the missing number: $\square - 6 = 8$

Type a number.

Practice 5

Find the missing number: $13 = 4 + \square$

Type a number.

Practice 6

Find the missing number: $5 + 5 = 4 + \square$

Type a number.

Practice 7

Mia has 8 stickers. She gets some more. Now she has 13. How many did she get?

$8 + \square = 13$

Type a number.

Practice 8

Find the missing number: $15 - 5 = \square + 3$

Type a number.