How to count mean

Hello, I have a have a question. By Google Forms people answered for my questionnaires to me.
The question was: How many posts do you write on Facebook every week?
The answers:
1) 0-2 posts - 26.3% (5 people)
2) 3-4 posts - 47.4% (9 people)
3) 5+ - 26.3% (5 people)

From all of this - how do I count the mean? (a someone in a same research - but other results succeeded to count and say the avarage is "4" - but how we can count if we don't know the accurate results? - we know for example that people write 0-2 posts but we don't know the accurate numbers - if people write 1 post or 2 posts, we just know range.



Not a robit
You may have been able to come up with something, but the 5+ seems so open-ended. A person could have had 100, which make the mean potentially off centered.

@GretaGarbo - any suggestions?
Did you ask me? :)

I have seen a standard method is to take the middle of the interval but I don’t remember how to treat the open interval (the 5+ interval).

I was (like Dason) thinking about using the censored Poisson distribution. But maybe it does not fit.