Another solution is to provide an overview of the trade-offs involved, with some example routes and links to useful tools. It's just as likely to address the OP's question and will be more valuable to other users who may have a similar problem in the future. If you see some option missing or perhaps want to provide other details, you can also post another answer to complement those that already exist.
The platform is based on the notion that a single canonical answer should be accepted but travel.SE has always been a bit more open-ended. No need to treat this as a mathematical problem or a competition in which only one answer needs to be the right one. IMO, if several partial answers have been posted, it's also fine to post a summary building up on them.
In fact, we can easily do that for all questions, even those that merely about “cheapest” or “fastest”, perhaps with a different emphasis depending on the specific criteria mentioned. For example if someone asks for cheapest and said the train is too expensive, mention cheaper train fares and hitchhiking and focus on the bus or car-sharing services, if someone asks about cheapest and fastest with no comparison point, propose a reasonable trade-off and mention other alternatives like busses, etc.
As far as the question itself is concerned, we can some ways to reformulate it or simply do nothing and let it be. As an example, consider Cheapest and fastest land ways to travel from Cologne to Prague? On one level the question is less than stellar, it does not specify many requirements and you can't really find a single ‘optimal’ solution or basis to claim that “my answer is better than yours”. But we now have answers covering all the main options and the whole thing is likely to be very useful to future readers. Nothing to be ashamed of or overly concerned about.
We can always request more details or suggest improvement but agonising over details of the formulation (“cheapest and fastest” is not materially different from “cheapest in a reasonable time“) or nitpicking over the choice of words when the intent is clear is not constructive. It's a bit like the perennial “walk or hitchhike“ comeback when someone asks about the cheapest way to go somewhere. We can try to nudge people into spelling out their requirements more explicitly but it's not unreasonable to make assumptions, that's how language works.