The equals sign means both sides have the same value. To find a missing number, use the inverse operation: undo adding with subtracting, and undo multiplying with dividing. Check by putting your number in the box and reading the sentence. When there are numbers on both sides, work out the side you can, then make the other side match it.
Explore this idea with maths tools
Maths you can move
Build 8 + 5
Tap two gold counters to fill the ten-frame.
8 in the frame + 5 waiting. The total stays the same.
Words to know
- equals
- has the same value as
- inverse
- the opposite operation that undoes another
Worked examples
What number goes in the box? 8 + □ = 15
- 1.The box goes with 8 to make 15.
- 2.Use the inverse: 15 − 8 = 7.
= 7
What number goes in the box? □ × 6 = 42
- 1.How many lots of 6 make 42?
- 2.Use the inverse: 42 ÷ 6 = 7.
= 7
What number goes in the box? 8 + 5 = □ + 6
- 1.The left side is 8 + 5 = 13.
- 2.□ + 6 must also make 13, so the box is 13 − 6 = 7.
= 7
For parents
Method: Use the inverse
What to say: What does the equals sign mean here? How can you check your number?
Common mistakes:
- Writes the number after the equals sign in the box, reading = as 'the answer is'.
- Uses the operation shown on the two numbers it can see, instead of the inverse.
- Counts on (or back) to find the gap but counts the starting number too, so the answer is one too big.
- Treats a multiplication or division sentence as addition or subtraction (finds the difference instead of the factor).
- For a − □ = c, adds the two numbers instead of finding how many were taken away.
- Reads = as 'the answer comes next' and writes the total of the left side in the box.
- Keeps calculating from left to right, using every number shown (8 + 5 = 13, + 6 = 19).
- Notices one number changed but adjusts the other in the same direction instead of the opposite one.