Question
Let `n` be a natural number. Prove that if the sum of the digits of `5n` is equal to the sum of the digits of `n`, then `n` is divisible by `9`.
Difficulty level (1 very easy - 10 very hard): 3
Topics:
Number Theory
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Modular Arithmetic / Remainder Arithmetic
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Divisibility Rules
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Divisibility Rules by 3 and 9
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