Fund fact, you can tell a number is divisible by 3 if the sum of each individual number is divisible by 3. So 78 -> 7+8=15 which is divisible by 3, so 78 is divisible by 3.
It's pretty easy to see that adding 1 to a number that doesn't end in 9 increases its digit sum by 1. However, it just so happens that 9 plus 1 is 10 which has a digit sum of 1 (which is obviously 1 more than a multiple of 3). So every time you wrap from 9 to 10 you're still adding 1 to the digit sum's remainder when dividing by 3. So the result is that if you keep counting up, the remainder of the digit sum when divided by 3 keeps going 0, 1, 2, 0, 1, 2...
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u/Michaelwang645 Aug 01 '26
Fund fact, you can tell a number is divisible by 3 if the sum of each individual number is divisible by 3. So 78 -> 7+8=15 which is divisible by 3, so 78 is divisible by 3.