Work out missing number instantly with clear inputs, formula shown and shareable results.
Each missing-number puzzle is solved by applying the inverse operation to both sides, then substituting the answer back as a check. Missing-number sentences are the first algebra a child meets: the box is a variable, and the inverse operation is the rule for isolating it.
Inverses
k + x = r gives x = r - k; k - x = r gives x = k - r; k x X = r gives x = r / k; k / x = r gives x = k / r
Check
Substitute the answer back into the original sentence and confirm it equals the result
k + x = r gives x = r - k; k - x = r gives x = k - r; k x X = r gives x = r / k; k / x = r gives x = k / r. Each missing-number puzzle is solved by applying the inverse operation to both sides, then substituting the answer back as a check.
Missing-number sentences are the first algebra a child meets: the box is a variable, and the inverse operation is the rule for isolating it.
Enter operation, known number, result. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.