Try this one........................

1)There are seven people in a elevator.The lift stops at 3(unspecified) floors.In each stop or more people get out, but no one boards it.After 3 stops it is found that the elevator is empty.Find the no. of way this can happen..................

Ans...1806

2)There are 100 people waiting to enter a hall in which the seats are numbered 1 to 100.The 1st person enters and takes any seat of his choice.There on each person enters the hall and sits in corresponding numbered seat(eg.2nd person enters and takes seat-2) IF AVAILABLE, AND IF NOT THEN TAKES ANY OTHER SEAT.Find the no. of ways the arrangement of the persons in the room is possible.................
Source:ISI(BStat 2010)