One day, the security guard counts 100 people entering the building in the morning but only 93 leaving at night. He knows that the remaining seven people could not have disappeared, so he investigates the building and finds out that the chairman has called an important board meeting in the conference room.
The security guard does not want to interrupt the conference, so he assumes that the remaining seven people are in the board meeting and goes home. Next time, however, the guard decides that he will need a way to know exactly how many people are in the conference room at a time in order to keep the building completely safe.
