Here's the original problem:

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Let R be the relation congruence modulo 4. which of the following equivalence classes are equal?

[7], [-3],[5],[-27],[32],[-14],[243],[9]

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I first began the problem by setting up four classes: 0, 1, 2, and 3. I then divided each number, for example 7 divided by 4 which yielded a remainder 3. So, 7 would be in class 3. I know how to do the positive numbers, but how would I go about doing the negative?