"The long dresses"
Translation:Les robes longues
Well in my opinion only "les longues robes" is correct and "les robes longues" shouldn't be accepted.
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Sitesurf confirmed on another thread (https://www.duolingo.com/comment/467855$from_email=comment) that there is a difference in meaning between "les robes longues" and "les longues robes". With "longues" in front of "robes" (conforming to BANGS), the adjective is a subjective attribute. In the reverse order, the adjective is a factual attribute, which is to say "les robes longues" is a term used by the clothing industry to describe a type of garment -- full-length dresses down to the ankles.
The English statement, on the other hand, is ambiguous in this regard. So, both translations are accepted.
Usually « grand(e)(s) » comes before the noun. I'm not sure if you're allowed to put it after the noun in this case. I also get the feeling it would mean "The big dresses" i.e. dresses for big women, rather than "the long dresses", even though in the right context « grand(e)(s) » can mean "long".