The issue at hand is whether it is always necessary to give complete answers, or whether there is value in getting the OP mostly there, and then leaving it up to the OP to "bring it home".
I personally very much liked Poutink's "teach a man to fish" approach here . In this particular case, I thought the OP was much better served by being provided the equation, and then left to plug in the numbers on their own, rather being provided a complete answer. I.e., I think this OP would have particularly benefited from that exercise. My only suggestion for Poutink would be to have made his pedagogic approach explicit to the OP (or maybe he did this and I'm forgetting), saying something like:
"Here's the equation. I think it would be a good exercise for you to try plugging in the numbers on your own, being careful to both keep track of units and follow the significant figures rules explained in the comments. When you're done, let me know what you get in the comments for the numerator, denominator, and overall, and I'll let you know if it's right or if you need to try again."
Or even just: "I've left it as an exercise for you to plug in the numbers on your own."
Indeed, I've followed that pedagogic approach in some of my own answers. E.g., here I only corrected the error, and indicated to the OP I was leaving it to them to rework the derivation on their own: How is the formula of mean activity coefficient derived?
Granted, the OP only asked that the error be identified, so I suppose you could say this case is different b/c here merely correcting the error did represent a complete answer. But the point is that my philosophical approach was the same as Poutink's.
Here's another instance in which I gave the person guidance on how to figure out the answer on their own, rather than doing all the work for them:
Combound with pressure sensitive melting point at moderate temperature and pressure?
MaxW also asks, more broadly: "What is a 'good answer'?" I think the characteristics of a good answer depend on the nature of the question.
If the OP is confused about something, the best answers are those that are able to "see into" the OP's question, determine the essential source of their confusion, and craft an answer that directly addresses that confusion. Often this requires going beyond the OP's literal request, since the OP may not themselves realize the fundamental source of their confusion—i.e., they may be asking about concept A, not realizing the reason they're confused is that they have concept B wrong.
If the question is more general, the best answers are those that, while addressing the OP's request, go beyond that to serve as a nice reference on the subject for the community as a whole.
 "Give a man a fish and he can eat for a day. Teach a man to fish and he can spend all day drinking beer."