What is an unknown?
Sometimes a number in a sum is hidden. We use a letter (like a, n or x) to stand for that hidden number. The letter is called the unknown.
Key idea
A letter is just a number in disguise. In n + 3 = 7, the letter n is the number we want to find.
Finding the unknown
To find the letter, do the opposite operation. If something is added, take it away. If something is taken away, add it back.
Worked example
Find n in n + 4 = 9.
Take 4 away from both sides: n = 9 − 4 = 5.
Check: 5 + 4 = 9. Correct!
Letters in word sums
If a box holds p pencils and we add 2 more, we write p + 2. Letters help us write a sum before we know the number.
Remember
- A letter stands for one unknown number.
- Use the opposite operation to find it.
- Always check by putting your answer back in.