What did it spit out? Because there is more than one solution (or none, if unhelpfully pedantic).
It's certainly possible with practice to quickly mentally figure out that 3.75 * 4 is the least whole multiple, so n = 4. Quick and dirty 2,2 makes a possible median and so 1, 2, 2, 10 and 2,2,2,9 are clearly solutions, since x+y= 15-4 where 0<x≤2, y>2 is not hard to do mentally. I struggle to call this a hard or enormous mental calculation. (Not missing the other solution, just acknowledging it takes an extra step - apparently it is hard for ChatGPT!)
4o actually struggled with keeping a median straight: https://chatgpt.com/share/67225a67-48ec-800f-a581-9651e7f65d...
And o1-preview had a very difficult time coming up with all solutions. https://chatgpt.com/share/67225ead-c9bc-800f-bcc1-067aacbf2b...
I'm not terribly impressed by that last one especially for nearly 2 minutes of "thinking". I also don't love that it keeps saying, set, set... A helpful tutor should at least gently point out these are not strictly sets. There's no solution of a set of positive integers that solves that problem.
If your GPT didn't give those 3 solutions right off the bat (I could not get mine to) and clarify they are not formally sets, it is a type of failure. Sure you can argue that a better prompt would improve it, but this is a problem, non experts putting in ill-formed or flawed prompts and not getting useful feedback where an expert human would be more helpful. A good human will tell you it's a multiset and in general there isn't a unique solution so the problem could be phrased better.
I asked o1-preview to make the question more rigorous and it came up with "Find all sets...".