why had "6 mètres de longeur" been given as wrong?

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Bonjour Nicholas,
Since the complete response was not posted with your question, I imagine that the incorrectness of the response had to do with the verb choice used.  If possible, would you repost the complete response phrase to give a better example of your concern.
Merci en avance,

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I had the same issue.  I think the quiz needs updating to bring into agreement with the lesson.

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Same here. The ‘fait’ was already given. I can only think it didn’t like ‘six’ when the question gave ‘6’

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I also believe that the lesson says that “longueur” should be acceptable. Also, on a non-language point, “six” is rejected because the answer requires “6”, which seems harsh.