Section 3.2: 3,13,16,20,22,25
  
Extra Credit: 17 (just like 16)

Hints: 

For 13 - remember the formula (3.1)
For 25 - think about how you did 20

  The nondistinguishable case is like problem 22.

  In general, you could think of it like this: After each 
  elevator run, you are given a sheet of paper which only 
  contains the info of how many people got off at each floor.
  The sample space is the total number of possible sheets of
  paper, the event of interest, those in which the record 
  reads as a list of 0's (no-one on that floor) and 1's 
  (1 person left at that floor). The distinguishable case
  adds has the names of the people as well.