French language Q&A Forum

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

Thinking that I understood the rule,I stupidly wrote "Oui c'est bon,"  and of course got it wrong. My natural inclination was to put "oui, c'est bonne." But, I veered because of the rule. Obviously, I do not understand the rule. Would you please explain it more clearly? :o)

Why is there "le" in the sentence: Tu révèles ton secret et nous révélons le notre. 

Is there a general rule in French about when to spell out numbers? For example in american english I think the rule is to spell out numbers ten or less and just use the number when greater than ten (eg, 12 instead of twelve). Thanks!