Deleting a question will automatically delete all the answers posted on that question, irrespective of the number of upvotes each answer received.
That is why in the moderator tool guidelines, users are asked:
Before voting to delete, please check whether there are any good answers; if so, then the question should be flagged for moderator attention as a potential merge candidate. We don't like to lose great answers!
Also, be cautious when deleting questions closed as duplicates; they can serve as a signpost, directing users to useful answers on another question.
It takes 3 votes, minimum, to delete a closed question. However, the number of delete votes required scales to the number of votes on the question and all its answers.
I have seen many great answers — but this is a very subjective area — that have earned only a handful of upvotes. Answers which have earned 50 or more upvotes are not necessarily "great", but they may have been perceived to be the "right answer".