Versions of this question have been asked before (check questions tagged deleted-questions), but the problem persists.
A new user posts a question, obtains an answer, then promptly deletes the question, before the answer gets upvoted (which would have prevented deletion of the question). Presumably, the question is a homework exercise, and the user deletes the question to hide their tracks. This is not only annoying, but also insincere, a form of cheating.
A simple way to solve this problem would be to prevent users with low reputation from deleting an answered question, even if the answer has no upvotes. For all users, I would suggest alerting an answerer if their answer got deleted in this way, and allowing them to undo the deletion (perhaps with the intervention of a moderator).