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![]() See examples and videos for more details. The rule for divisibility by 3 is simple: add the digits (if needed, repeatedly add them until you have a single digit) if their sum is a multiple of 3 (3, 6, or 9), the original number is divisible by 3: Take the 1 and the 5 from 15 and add: Now you try it. ![]() is included just because it makes an otherwise complicated proof easy. ![]() Note that a certain amount of algebra is required when completing proofs. The subject of proofs, how to decipher them and why we need them, comes next.
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