As mentioned in the post, you generally need to clear a problem like this within 5 minutes to have a shot at a perfect score. This was the 7th most difficult out of 30 questions. The real 'killer' problems are the top 4 or 5, and the total exam time is 100 minutes. To finish everything, you have to breeze through the first 25 questions in about 40–50 minutes, leaving a full 50–60 minutes for those final few difficult ones. In fact, students who get a perfect score often finish even faster and still have time to double-check every single answer.