Complete number sentences (find the unknown) - One-step
Theory
The equals sign means “the same value as”, so both sides of a number sentence must balance. Find the missing number by undoing the operation, then check by putting it back.
A number sentence is a statement such as \(\square + 12 = 30\).
The equals sign tells you both sides have the same value.
The missing number makes the sentence true: here \(\square = 30 - 12 = 18\).
Which operation undoes which
| Operation | Undo with |
|---|---|
| \(+\) | \(-\) |
| \(-\) | \(+\) |
| \(\times\) | \(\div\) |
| \(\div\) | \(\times\) |
How to find the missing number
- Read the sentence and spot the operation.
- Undo it with the inverse operation.
- Check by substituting your answer back in.
Subtract to undo the \(+12\). Check: \(18 + 12 = 30\).
| \(30 - 12\) | \(=\) | \(18\) |
Divide \(40\) by \(8\). Check: \(40 \div 5 = 8\).
| \(40 \div 8\) | \(=\) | \(5\) |
Divide to undo the \(\times 8\). Check: \(8 \times 4 = 32\).
| \(32 \div 8\) | \(=\) | \(4\) |
The \(14\) is the total, so subtract.
| \(14 - 6\) | \(=\) | \(8\) |
Common pitfalls
Frequently asked questions
What does the equals sign mean?
It means the two sides have the same value. It is not a signal to write an answer next.
How do I find a missing number?
Undo the operation with its inverse. For \(\square + 12 = 30\), subtract: \(30 - 12 = 18\).
How do I check my answer?
Put it back into the sentence. If both sides are equal, it is correct.
What if the box is in a subtraction?
Use the matching inverse. For \(\square - 5 = 9\), add: \(9 + 5 = 14\).