Probability that no two people get off elevator on same floor Seven people enter the elevator on the first floor of a $12$ story building. What is the probability that no two will get off on the same floor? You may assume that all floors are equally likely and that no one gets off the elevator before it starts up.
I believe that answer is $\frac{11P7}{11^7}$, but the book lists the answer as $\frac{12P7}{12^7}$. Which is correct?

The problem statement specifies that there are 12 stories, so there are 12 other floors to go.  Therein lies the misunderstanding...
