6 . The traffic lights at three different road crossings change every 21 seconds, 63 seconds and 105 seconds respectively. If they all change simultaneously at 9 : 15 : 00 AM, at what time will they again change simultaneously?
7 . In an examination a student scores 7 marks for every correct answer and loses 1 mark for every wrong answer. If he attempts all 150 questions and scores 400 marks, the number of questions he attempts incorrectly is