When you multiply me by 2, the product is 15 less than the value of my square. If I am a positive integer, what number am I?
5
Set up the equation 2x = x^2 - 15. Rearranging into standard quadratic form gives x^2 - 2x - 15 = 0, which factors into (x - 5)(x + 3) = 0. Since the number must be positive, x = 5.
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: Easy. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
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