A high school drama club finds that they can sell 500 tickets to the school play if they charge $5.50 for each ticket. A survey of students indicates that for each dollar the ticket price is lowered, the average number of tickets sold will increase by 100.
Write a linear model which expresses number of people who purchase tickets (N) as a function of the price of a ticket (P). What is the physical meaning of the slope in this linear relationship?