7 passengers enter an elevator on the first floor of an 11-floor building.
Assume that the egress pattern to floors 2,3, ... ,11 is the same.
What is the probability that all get off at different floors?
Each of the 7 people have 10 choices of floors on which to exit.
. . There are: . possible outcomes.
How many ways can they exit on different floors?
There are: . ways.