I'm reading a chapter on linear congruences such as

What is confusing me is this sentence, "we may ask how many of the m congruence classes modulo m give solutions; this is exactly the same as asking how many incongruent solutions there are modulo m".

Why are the congruence classes that give solutions considered incongruent? It would seem to me that is the opposite: they should be congruent solutions.

The example: Find all solutions to

. A complete set of three incongruent solutions is given by x=8, x=13, x=3. I understand how to calculate those three values of x, I don't understand why they are calling these "incongruent solutions".