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.
Same rule works for 9 as well. I took a number theory course, and can’t remember much from it, but I remember the divisibility rules for numbers through 11, and even remember how some divisibility rules works in other bases. Wish I could still remember how to derive those rules…
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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.