What is an unknown?
An unknown is a letter that stands for a number we do not know yet. We often use letters like a, x or n. For example, in 5 + n = 8, the letter n hides a number. To find it, we ask: "5 plus what gives 8?" The answer is 3, so n = 3.
Key idea / 要点
A letter takes the place of a number. To find it, use the opposite operation: undo + with −, and undo × with ÷.
Finding the unknown
If we know y − 4 = 10, we add 4 to both sides: y = 10 + 4 = 14. If 3 × k = 18, we divide: k = 18 ÷ 3 = 6.
Worked example / 例题
Solve p + 7 = 12.
Subtract 7 from both sides: p = 12 − 7 = 5.
Check: 5 + 7 = 12. Correct!
Remember / 记住
- A letter is just a "mystery number".
- Use the opposite operation to find it.
- Always check your answer by putting the number back.