if "su" hasn't been identified it can be confusing in spanish just like he and she in english so this spanish sentence itself can mean three things but to make it clearer you can say "ella va a reducir el porcentaje de usted" or "ella va a reducir su propio porcentaje" or "ella va a reducir el porcentaje de él"
i hope that makes more sense
I just read the other day that "ir a" plus infinitive is used more in Latin America to indicate a future activity, whereas the future tense conjugation is used more in Spain. Duolingo will accept either one (in my experience). If they do not and you feel they should, report it as an error.
Darn it, I had a feeling someone would ask me that. HonestIy, I can't remember. I'll try to find it and if I do I'll come back and edit this post.
EDIT: Rickydito - I found it. Can you believe it, it was in a Duolingo discussion thread: http://www.duolingo.com/comment/935823. Here is another thread from stackexchange that discusses the differences between the two forms of expressing the future, regional differences as well as others: http://spanish.stackexchange.com/questions/319/ir-a-infinitive-vs-future-tense. It seems there are a lot of opinions on this topic. :)
Actually, "going to" in English refers to the future period, either near or distant,e.g. "I'm planning to retire when I turn 80", especially if spoken by a 20-year-old! My point was that when both English and Spanish offer the very same construction of "going to", I would expect they'd want the strict translation. In other words, for the very reason you state, namely that both languages use both constructions for any reference to a future event, I expected they'd want the answer that employed the same construction. They really should try to avoid ambiguity to avert this kind of confusion.
Finally figured this out after reading http://www.studyspanish.com/lessons/possadj.htm "If the meaning of su is not clear from the context of the sentence, a prepositional phrase is used in place of su."
So the correct answer would be (If I'm correct) "Ella va a reducir el porcentaje de ellos."
To elaborate on your source and what I have learned is that because the prepositional phrase was not included - su=her. This is because 'ella' is the only actor introduced in our given context. If somebody or something else is introduced into the context in a previous sentence or by the prepositional phrase, then it is one of the other relevant options for su mentioned in this discussion thread.
In this circumstance "percentage" is awkward and usually would not be used without a "of _." Common usage in business, something that is easier on the ear, is "She is going to reduce her share".
A literal translation does not always produce correct usage in the second language.
If your real estate agent normally charges 4% of the closing cost of the house as her fee, and chooses to reduce it to 3% because she likes you - or is sharing the fee with a second agent - then, when she does sell your house in the future, "Ella va a reducir su porcentaje." :) From a native-speaking former real estate agent. :)