I suspect there are a number of cases of posts on here by students whose first
language is not English, and they are translating their problems into English,
as best they can.  I would suspect this to be the case here upon seeing this
wording:

So perhaps we tutors should be a little nicer since we don't know the whole 
story behind these students and their posts.

Although I agree with the other tutors that the instructions here are
insufficient, I think there should be problems just like this one where the
answer is not a number, but an expression in terms of literal quantities chosen
by the student.  For all we can tell, the instructor intended the problem to be
done this way, and he/she made those instructions clear in class.

Perhaps the student was supposed to solve the problem this way:
 
Let r = the number of marbles that were in the first bag.
Let s = the number of marbles that were in the second bag.
Let t = the number of marbles that were in the third bag.
So there are r+s+t marbles in the bowl.

Therefore we have the following:
The number of yellow marbles that were in the first bag = 0.36r.
The number of yellow marbles that were in the second bag = 0.32s.
The number of yellow marbles that were in the third bag = 0.25t.
So there are 0.36r+0.32s+0.25t yellow marbles in the bowl.

Thus the fraction of yellow balls in the bowl is given by the
expression:

  <---answer

Edwin