What number multiplied by itself minus two equals twenty-one?
Category: Math · Difficulty: hard
Hint: Think about the quadratic relationship in the statement.
Answer
Five is the correct number
Explanation
If x(x) − 2 = 21, then x² = 23, which is not a perfect square, so the original meaning is ambiguous. Check closely: Possibly the problem meant 'a number multiplied by itself, then subtract two, equals twenty-one.' Then x² − 2 = 21 → x² = 23, no integer solution. So if it means 'a number multiplied by (itself minus two)', so x*(x−2) = 21, then x² − 2x − 21 = 0. Solve quadratic: x = (2 ± √(4 + 84))/2 = (2 ± √88)/2, no integer solution. Another interpretation: number multiplied by itself minus two — could be 'number × (itself − 2)'. Let's rewrite the riddle for clarity; or choose another riddle to avoid confusion.
This is a hard math riddle — a tricky one that takes a little thought. The answer is Five is the correct number.
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