"Jeff is not a swimmer", symbolically I have to turn this into symbolic language: > A: No swimmers are overweight > B: Jeff is overweight > Therefore, Jeff is not a swimmer I got $$A\to\neg B$$ $$B$$ $$\therefore\neg A$$ What do you think? Am I close? Also I would say this is valid via universal modus tollens.

What you have is correct, where $A$ means "swimmer" and $B$ means "overweight".

One thing to point out: You translated "no swimmer is overweight" as $A \to \lnot B$. That is fine, but a more direct translation might be $\lnot (A \land B)$. I would think of "$A \to \lnot B$" as "every swimmer is not overweight", and "$\lnot (A \land B)$" as "no swimmer is overweight." But of course the statements are logically equivalent, so it doesn't matter too much. Just pick the one you think is a more direct translation.