"I have thirty pairs of earrings."
Translation:J'ai trente paires de boucles d'oreilles.
I am not vast in French yet, but I think for all those words (adjectives of quantity) that one uses to express amount of something, "de" has to follow them. I don't know of an exception yet. For example; Trop de soucis = Too much worries, Beacoup de gens = A lot of/Many people, Un peu de sucre = A bit of sugar, Un morceau de fromage = A piece of cheese. In this Duo's exercise, even though the word 'paires' is in plural, it still has to be "de" that must be used and not "des" because "paires" is doing a job similar to those of "trop" "beacoup", "peu" and "morceau" in the examples I gave above. The "paires" only gets pluralized to agree with "boucles d'oreilles".