Imagine that you and 99 other people were invited to participate in a bet. All of you have to form a line, provide a one dollar bill on which all of you write your number in the line (from 1 to 100). Host will collect all of your one dollar bills and take them to a room with 100 boxes (each box has its own number (from 1 to 100)). Host will place randomly distribute your bills among the boxes. After he has done this, everyone of you one by one will come into the room and look for your own bill, but you will be able to check only 50 boxes. If every player will find his own bill, then you all will win a game - you will regain your dollar as well as you will additionally get another 100 dollars. But if at least one player makes a mistake, you all will lose. So would you sign up for such bet?
Please note that there are certain restrictions. You are able to communicate with other players only before the game. Together you may form a strategy. During the game only one person will be in the room at a given time. That person will have to leave room in its original state (so no tampering with the system - no markers, moving boxes or bills and etc.).
Think a bit and then watch the following videos. First one explains the rules of the bet, while second one provides an answer.