What is an unknown?
An unknown is a number we do not know yet. We use a box ◻ or a letter to stand for it. Our job is to find its value.
Key idea
To find an unknown, do the opposite operation. If it was added, subtract. If it was subtracted, work backwards to undo it.
Worked example
Worked example
Find ◻ if ◻ + 5 = 12.
We added 5, so do the opposite — subtract 5:
◻ = 12 − 5 = 7
Check: 7 + 5 = 12. Correct! So ◻ = 7.
Always check
Put your answer back into the sentence to make sure both sides are equal.
Remember
- An unknown stands for a missing number.
- Use the opposite operation to find it.
- Check by putting your answer back.