"The athletes are slow and weak."

Translation: A sportolók lassúak és gyengék.

[JMaxGlobal]

As "sportolók" and "atléták" are both words for "athletes", I think my answer:
"Az atléták lassúak és gyengék" should have been accepted. I'll let the DuoLingo team know, and see if they agree that this can be an alternate translation.

[vvsey]

"Atléták" may be more specifically used in Hungarian for people who practice the types of sports under the umbrella of "athletics". That is, track and field and such. The more generic, more encompassing, term is "sportolók". With that in mind, I think both could be accepted.

[AlexPotter87]

Are both the adjectives supposed to be plural in such a scenario or only the first one?

[ChrisCrios]

Both should be plural since they are behind the adjective. If you place them in front of sportolók they are singular, but this means something like 'the slow and weak athletes', i.e. the slowness and the weakness become a classifying property of the athletes.