I am thinking of a positive integer. If I square it and then subtract 9 times the original number, the result is 10. What is the number?
10
Let the number be x. Equation: x^2 - 9x = 10. Rearrange to standard quadratic form: x^2 - 9x - 10 = 0. Factor: (x - 10)(x + 1) = 0. Since the number is positive, x = 10.
About this riddle
Math riddles hide a calculation inside a short story. Pull out the numbers, work out what is being asked, and watch for a trick such as a unit change.
Difficulty: Medium. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
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