I understand from threads I've seen on several exercises that it's ok if people include the 'v' even when grammar says it shouldn't be there. But I don't understand why I'm frequently marked as incorrect for dropping it when the grammar says I should. That just seems like a bad editorial decision on Duolingo's part and is confusing for us. My answer to this sentence was "η περιφέρεια είναι μέσα στη ζώνη" and Duolingo marked it as wrong and corrected it to "Η περιφέρεια είναι μέσα στην ζώνη." I've reported it, but just flagging here also.