The Tyre

At Sydney University, there were four students taking Organic  Chemistry. They did so well on all the quizzes, midterms and labs, etc., that each had an "A" so far for the semester. These four friends were so confident with the finals approaching that the weekend before, they decided to go down to Canberra and party with some friends there. They had a great time.

However, after all the hard partying, they slept all day Sunday and didn't make it back to Sydney until early Monday morning - the morning of their final exam! Rather than taking the final then, they decided to find their professor AFTER the exam and explain to him why they missed it. They explained that they had gone to Canberra to do some research in the ANU archives for the weekend with the plan to come back in time to study, but, unfortunately, they had a flat tyre on the way back, didn't have a spare, and couldn't get help for a long time. As a result, they only just arrived now! 

The professor thought it over and then agreed they could make up their final exam the following day. The guys were elated and relieved. They studied hard that night - all night - and went in the next day at the time the professor had told them. He placed them in separate rooms and handed each of them a test booklet, and told them to begin. The first problem was worth five points. It was something simple about free radical formation. "Cool," they all thought in their separate rooms, "this is going to be easy." Each finished the problem and then turned the page. 

Question 2 (for 95 points): Which tyre?