When you multiply me by any positive integer, the sum of the digits of the resulting product—when repeatedly summed until one digit remains—is always me. What single-digit number am I?
9
The digital root of any multiple of 9 is always 9 (for example, 9 × 7 = 63, and 6 + 3 = 9).
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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