When the question "Je suis partie ___ jours" (I left for about fifteen days) is encountered in a quiz it may not be originating from this lesson yet the answer "pour environ quinze" is not accepted. Shouldn't it be? Aren't both this and "pour une quinzaine de" equally valid French here?
I can find examples of the environ version on the web including here:
http://context.reverso.net/translation/english-french/about+fifteen