The Final Exam

At Duke University, there were four sophomores taking chemistry and all
of them had an "A" so far. These four friends were so confident, that
the weekend before finals they decided to visit some friends and have a
big party. They had a great time, but after all the hearty partying,
they slept all day Sunday and didn't make it back to Duke until early
Monday morning. Rather than taking the final then, they decided that
after the final they would explain to their professor why they missed
it. They said that they visited friends but on the way back they had a
flat tire. As a result, they missed the final. The professor agreed
they could make up the final the next day. The guys were excited and
relieved. They studied that night for the exam. The Professor placed
them in separate rooms and gave them a test booklet. They quickly
answered the first problem, worth 5 points. Cool, they thought! Each
one in a separate room, thinking this was going to be easy. Then they
turned the page.

On the second page was written....

For 95 points: Which tire?________________