In a group, what percentage of members speak both french and English?

Thanks Bunuel for replying
I've already checked all those links and both formulas and I feel pretty confident with set questions.
So I can assume if I deal with DS set questions on test day, the question should always precise if there is someone in {Neither} group OR answer is E, right?
I was almost sure that GMAT doesn't trick candidates in that way...

I wouldn't call this a trick. This is natural and logical: of course there could be a group which does not speak any of the two languages, why not? Thinking otherwise would be assumption.

Since Bunnel has posted there is no point in posting again but still this is my take
statement 1
insufficient . we don't know the percentage of french
statement 2
We don't know the percentage of English
insufficient
combine statement 1 and statement 2
when we have overlapping sets , it is necessary to know the condition if anything satisfies both the individual statements or if it satisfies neither .
Here 2 cases could be possible :
1. members who speak English also speak french or vice versa ( how will we know the information from given statements- insufficient)
2. members neither speaking English or french (we don't know about it either. hence insufficient)

hence the answer is E

awesome thanks. I got confused because I thought that in terms of GMAT, the "overlapping" nature of sets - it's an additional condition, that should be clarified in the question and if not shouldn't be taken into account. I mean, I can develop this logic further, we don't know if any member speaks any language but english and french. I mean that kind of problem is not an algebraic expression or calculus or geometry where you have clear although wide framework to operate within it and you don't have to make any assumptions that contradict that framework.
i will try to cover more set questions to catch on

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