To find how many different ways can someone enter then leave the auditorium, we need to find how many ways can someone enter, how many ways can someone leave, and then multiply these possibilities:
Number of ways that someone can enter: 4 (3 doors are only entrance and 1 door is entrance and exit)
Number of ways that someone can leave: 2 (1 door is only exit and 1 door is entrance and exit)
So the number of different ways that someone can enter and leave the auditorium is 4*2 = 8
