6. In a mathematics competition, the first round consists of 25 questions. According to the marking rules, each correct answer earns 4 points, and each wrong answer (including unanswered questions) deducts 1 point. If a score of no less than 60 points qualifies a student for the second round, then, how many questions at least must a student answer correctly in the first round to qualify for the second round? $\qquad$