Let us turn it around, what does "Question on probability" convey?
The "Question on" is redundant. Everything here ought to be a question. It is not as if we had different types of threads and you had to signal this one is about a question, as opposed to being a review of a book, or announcing a seminar, etc.
Further, the "probability" is redundant with the tag probability.
Thus, no additional information is conveyed by this title, put bluntly, the title is useless.
You say that you do not see how you can include more information in the title without just posting the entire question. But there is no problem with posting the entire question in the title. Just do that.
Then make sure to repeat the question in the body and do not forget to add some more context.