I think if I were in that situation I would look at the syllabus to see if it leaves any ambiguity. There's a good chance that the grade you received is not covered in the syllabus, as it may very well dictate that 80-89 is a B and 90-100 is an A, but it does not specify what an 89.85 is. I've actually had a syllabus that read something to the effect of, "a grade in [80,90) is a B and a grade in [90, 100] is an A." Then again, we mathematicians are notoriously nitty...