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I'm trying to understand what mod means in this equation and how to solve it:

d * 13 = 1 mod 1680 

This is from how to make a public and private key pair. The answer is 517 apparently and I can get that from wolfram. I assume mod is %, but that doesn't seem to work out. I've also seen that this could me mod( 1, 1680 ) which supposedly equals

mod( m, n ) = m - n ( m / n ) 

But for that I get 1 and then 1 / 13 is obviously not 517. Just looking for some direction. Thanks.

Ha, I know so little that I can't even find a tag to add.

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    Compare [Math use](http://en.wikipedia.org/wiki/Modular_arithmetic) and [programming use](http://en.wikipedia.org/wiki/Modulo_operation).2011-11-09

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