Mean average with range of values as a value in frequency table

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Preface: This is for a homework for a Psychological Statistics class, the other questions are equally badly written or worse. I'm just trying to understand if there's something I'm missing. No good way around this from the lectures that I attend, the book, or the slides from class.

I've emailed my professor, but he has yet to get back to me. I don't understand how this question is possible to answer because the part where it says "11 or fewer" doesn't seem right to me. Is there something I missed?

I have a basic statistics problem set and one of the questions is as follows:

X__________f

15_________20

14_________20

13_________10

12_________50

11 or fewer___40

What is the mean of the values?

I know that the mean of some set of values (unweighted) is the sum of the values divided by the number of the values summed. This just confounds me. Should I just wait for the professor's response?