Sitemap priority ranges from 0 to 1. Based on your question, it would be fair to assume that a question without answers is a 0.
1. But what would be a 1? The question with the most answers?
2. What would be a 0.5? The question with half the answers of the question with the most answers?
Note the solution needs to be performant and take advantage of the indexes properly so it is not an option to do too much magic there