I lost my Boarding pass

100% because all the tickets are sold out and all the passengers for that flight have boarded teh plane

Nt getting..Plz explain.

When the last passenger boards the plane, there are just two possibilities: the one remaining seat may be his or that of the first passenger. Under the assumption that no preference has been exhibited by the boarding passengers towards either of the two seats, they both have the same probability to become the last unoccupied seat: 50%