At Yale 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 Old Eli 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 told him they

 Had 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 next day 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 separate rooms thought this was going to be easy ... then they turned the page. On the second page was written...      

For 95 points:     Which tire was it?