Rather than take the final then, they decided to find their professor after the final and explain to him why they missed it. They explained that they had gone to UVA for the weekend with the plan to come to study but, unfortunately, they had a flat tire on the way back, didn't have a spare, and couldn't get help for a long time. As a result they missed the final.
The Professor thought it over and then agreed they could make up the final the following day. The guys were elated and relieved. They studied that 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.
They looked at the first problem, worth 5 points, something simple about free radical formation. "Cool," they thought at the same time, each one in his separate room, "this is going to be easy."
Each finished the problem and then turned the page. On the second page was written: (For 95 points): Which tire?