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What is modulo arithmetic - Mathematics Stack Exchange
Similarly, your mod function mod (m,n) performs the equivalent operation, resulting again in 1/13. The original equation asks, $$13 d\equiv 1 \qquad (\operatorname {mod} 1680)$$ or, what number, when multiplied by 13, is equivalent to 1 mod 1680? The first thing to realize is that, if 13 does not divide 1680, there are infinitely many numbers.