Missing Number Equations: A Simple Guide
By the Duck Math team ·
A missing number equation has one unknown, shown as a box or a question mark. Solving it is a child's first step into algebra. The whole skill is knowing the inverse operation.
The big idea: undo the operation
Addition and subtraction undo each other. Multiplication and division undo each other. To find the missing number, undo what was done to it.
Addition
- 7 + ? = 15 → 15 − 7 = 8
- ? + 7 = 15 → 15 − 7 = 8 (the order does not matter for addition)
Subtraction (the tricky one)
- ? − 6 = 9 → 9 + 6 = 15 (add back what was taken away)
- 15 − ? = 6 → 15 − 6 = 9 (the missing number is the amount taken away)
Multiplication
- 6 × ? = 42 → 42 ÷ 6 = 7
- ? × 9 = 54 → 54 ÷ 9 = 6
Division (also tricky)
- ? ÷ 4 = 12 → 12 × 4 = 48
- 48 ÷ ? = 6 → 48 ÷ 6 = 8 (the missing number is the divisor)
Check by putting it back
Always substitute your answer. For 15 − ? = 6 with ? = 9, check 15 − 9 = 6. It works.
Practise
Math Duck is built around this skill: each level hides one number in an equation, and you must find it before you can open the door. The missing number solver shows the step for any equation you type.
Questions
Why use a question mark instead of x?
A box or question mark is friendlier for young children. Later, a letter such as x takes its place and the same inverse-operation method still works.