Missing Number Solver: Solve 7 + ? = 15
Find the missing number in an addition, subtraction, multiplication or division equation.
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How to use it
- Type an equation with exactly one ? in place of the unknown number.
- Use + − × ÷ (or x and /) between the numbers.
- Press Solve to see the answer and the inverse operation used.
How it works
Finding a missing number is early algebra. The trick is to use the inverse operation: if 7 + ? = 15, undo the addition with 15 − 7 = 8. For × and ÷ it works the same way: 6 × ? = 42 becomes 42 ÷ 6 = 7.
Subtraction and division need care. In 15 − ? = 6 the missing number is the amount taken away, so ? = 15 − 6 = 9. In 48 ÷ ? = 6 the missing number is the divisor, so ? = 48 ÷ 6 = 8.
Questions
Does it handle decimals?
Yes, numbers like 1.5 + ? = 4 work. Answers are rounded to six decimal places.
Why does 0 × ? = 5 fail?
Zero times any number is zero, so no number can make it 5. The tool tells you it cannot be solved.