Answer:
There are 7^20 possibilities fir the people to get off the elevator
Step-by-step explanation:
I am going to assume that some people (but not neccesarily all) get into the elevator in the low level and none at the highest. Since we are looking for the possibilities for the people to get off the elevator, we can assume that everyone get into in the low level, because this case will contain every possible scenario. Each person has 7 possible ways to get off, hence 20 person will have 7 possibilities powered 20 times, hence there are 7^20 possibilities fir the people to get off the elevator.