Translation:The postman is angry and sad: the newspapers are wet.
In this case, vizesek az újságok is the same as az újságok vizesek, just with rearranged word order. So the correct version is vizesek.
However, if the adjective is directly before the noun: "the wet newspapers" (a vizes újságok) (as in Heather131558's comment), then the adjective stays singular.
here is a related post : https://forum.duolingo.com/comment/16532192
Thanks; I guess it is like "Cheap, the hotel is?" or, as the French (speakers including Hercule Poirot) say "The hotel, is it cheap"?
Still, it is among the greatest mental contortions I've had to do so far; one gets very used to the Indo-European functional sentence perspective. I could live with something like "because it rained on the newspapers" but that would be phrased quite differently, I'm certain.