1) If the topics created to discuss a problem, it would be easy to make out it's title as: "Problem: <Short description of the problem>". In such atopic would be natural to find out what is the problem, how the problem is urgent, why it may be important, post links to topics of different solutions.
All topics with a problems should be posted to in-port...@googlegroups.com
group instead of in-por...@googlegroups.com
group. But topics about project itself should instead go to in-por...@googlegroups.com
2) If the topic was created to discuss some solution of the problem, it would be easy to make out it's title as: "Solution: <Short description of the solution>". In this topic would be naturally provide links to topics of problems that this solution helps to decide, describe in detail what is theessence of solution, and discuss how this solution is adequate for the linked problems. Also, a particular solution can be a source of new problems. If someone has a desire to discuss these issues, it probably makes sense for these problems to create separate topics, in accordance with paragraph 1) and make link references accordingly.
If there is a solution, then is posted to discussion where problem is described instead of creating a new discussion. Whatever topic is a problem or a solution is determined purely on group, where it's being posted to, so no additional prefix is needed.