Why don't you mind your own business?
 →  너나 잘하세요.

[예문]
A:Do you know that we have final exam tomorrow?
 너 내일 기말고사있는거 알지?

B:Of course i do.
  물론 알고있지.

A:Why don't you work hard?
 근데 왜 공부안해?

B:Why don't you mind your own business?
  너나 잘하세요.