I would recommend that, for each unique answer, players form pairs and both answer the question with the same answer while investigating different people. For example, if "1, 2, 3" is one of the possible correct answers, two people will answer with "1, 2, 3", and one will choose to investigate 1 while the other investigates 2.
In that case, for every answer permutation we are guaranteed to find an assassin if it turns out to be correct.
[MENTION=187]Laughing_gaara[/MENTION]
We seem to have similar answers, and I suspect you used the same method as I to derive them. Do recheck, though. I think the answer for "b" should be 14 and not 15. I'll double-check on my side as well. You're investigating 4 while I'm investigating 17, so either one of us gets an assassin, or none of us gets it and we can assume 14/15 is an assassin. If we do get the answer right, do tell me your investigation result so we can work something out.