The Nine-Digit Trick: Why Subtracting Digit Sums Always Yields a Multiple of 9

Here's a fun fact from number theory: take any whole number, subtract the sum of its digits, and the result is always divisible by 9. For example, 123: sum of digits = 1+2+3 = 6, 123 - 6 = 117, and 117 ÷ 9 = 13. This neat property holds for all integers, thanks to the magic of modular arithmetic. It's a classic puzzle that delights math enthusiasts and serves as a great introduction to divisibility rules. Next time you're stuck for a party trick, try it out!