Possible Duplicate:
Taking Seats on a Plane
There are 100 seats on a plane and 100 passengers, each with his ticket. However, the first person to enter the plane discovers he has lost his ticket, so he picks a seat at random. Afterwards, every new passenger sits in his place if it is free, and otherwise picks a vacant seat at random.
You are the last to enter the plane. What is the probability you'll sit in your seat?
I managed to solve this using induction (i.e. marking by $A(n)$ the probability where $n$ is the number of passengers and then finding a recursive formula for $A(n)$ which is quite simple). However, I want to know if there are more "instantly obvious" or one-liner solutions.
Let the first passenger to board have ticket for seat $p$, and suppose your ticket is for seat $q$. Either (i) $p$ is filled before $q$ is or (ii) $q$ is filled before $p$ is. In case (i), you will get seat $q$, and in case (ii) you won't.
These two cases are equally likely. For it is equally likely that the first passenger will choose $p$ or $q$. And if she chooses neither, then by symmetry $p$ and $q$ remain equally likely to be filled first, since they are the correct seat for none of the remaining passengers. So the required probability is $\frac{1}{2}$.