"She is nineteen and works with her mother."
Translation:Ela tem dezenove anos e trabalha com a mãe.
We don't. You could translate "a mãe" as "the mother" but in that case the sentence seems incomplete because it leaves us in the dark about whose mother she works with, and so the most satisfactory, if not almost obligatory translation is the one given. Despite what I say, the answer with "the mother" should also be accepted I think.