Without intruding, the security guard wants to find a way to determine exactly how many people attend these late-night meetings. He happens to notice that the temperature of the conference room goes up when more people are inside. He tapes a thermometer to the door and carefully measures the temperature of the room. When the conference room is empty, he measures the temperature to be 70 degrees and notes the temperature goes up by a degree for each person inside.
A week later, he counts 3 people missing at the end of the day. He checks the board room and sure enough, the door is closed and the thermometer reads 73 degrees. He knows
people are in the meeting. All 3 people are accounted for and the security guard is happy and goes home for the evening.
His formula for people entering and exiting the building is