Using multipliers seems like a generally-good approach, but I think is hard to express what I was suggesting above.
closed not solved < open not solved < open solved < closed solved
Closed should be a priority increase for solved posts but a decrease for unsolved ones.
Closed posts with no solution are nearly worthless — someone else had this problem, but there’s no answer for it here, and you can’t provide one. Closed post with solutions are, on the other hand, gold. They’re not merely solved, but definitively so.