Final Exam

This past fall semester, at Stanford University, there were two sophomores
who were taking Organic Chemistry and who did pretty well on all of the
quizzes and the midterms and labs, etc., such that going into the final,
they had a solid A.  These two friends were so confident going into the
final that the weekend before finals week (even though the Chem final was on
Monday), they decided to go up to Berkeley and party with some friends up
there.  So they did this and had a great time.

However, with their hangovers and everything, they overslept all day
Sunday and didn't make it back to Palo Alto until early Monday morning.
Rather than taking the final then, what they did was to find the professor
after the final and explain to him why they missed the final.  They told him
that they went up to The City for the weekend, and had planned to come back
in time to study, but that they had a flat tire on the way back and didn't 
have a spare and couldn't get help for a long time and so were late getting 
back to campus.  The prof thought this over and then agreed that they could 
make up the final on the following day.  The two guys were elated and 
relieved.  So, they studied that night and went in the next day at the time 
that the prof 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, which was something simple about free
radical formation and was worth 5 points. "Cool," they thought, "this is
going to be easy." They completed that problem and then turned the page.

They were unprepared, however, for what they saw on the next page.

It said:  '(95 points) Which tire?'